This list is a little old, so some of the links may not work anymore, but overall it's still a pretty solid compendium for any budget concious Linux (or Windows) gamer!
Know of a game that should be added to the list? Leave a comment below! ^_^
Counter Strike: Global Offensive, the premier online FPS [Free-to-Play]
No
Warfork
Fork of Warsaw to continue development, Cel-shaded Quake-like online FPS
CC-SA
Cube 2
Online Quake-like Arena FPS with multiple game modes & level editor
zlib
Assault Cube
Counterstrike-like FPS in the Cube engine
Freeware
Xonotic
Quake-like arena online FPS
GPL
FreeDoom
Open-source implemantation of Doom, best used with Chocolate Doom
Yes
Dystopia
Cyberpunk online class-based FPS
???
Urban Terror
Fast paced online FPS reminiscent of CS
???
OpenArena
Online arena FPS using the id tech engine
???
Fistful of Frags
Online Western FPS - Not as good as it used to be
???
Ravenfield
Singleplayer Battlefield type game
No
Alien Arena
Retro Sci-Fi Online FPS
???
Red Eclipse
Online FPS with parkour
SRC Available
ET Legacy
Wolfenstein: Enemy Territory for modern PCs
SRC Available
Team Fortress 2
Class-based online FPS with many game-modes to choose from (Not suitable for weak PC)
No
Team Fortress 2 Classic
Sick of hats, nerfs, and other changes to TF2? Than Classic is for you! TF2 Classic is a resurrection of what the game was like in 2013, with restored gamemodes, guns, and classes
No
Open Fortress
A total conversion of TF2 with unique characters, weapons, gamemodes, and maps!
No
Aleph One
Open Source implementation for Bungie's Marathon series
GPL
Double Action Boogaloo
Online 3rd-person shooter with slow motion
No
Ultrakill Prelude
Demo for an ultra violent oldschool FPS
No
OpenSpades
Clone of Ace of Spades 0.75, featuring fully destructible terrain and plenty of game modes
GPL
Blade of Agony
Story-driven FPS inspired by WWII shooters from the 90's and early 2000's - Extremely polished and fun
SRC Available
RPG
Name
Description
Open Source?
Daggerfall Unity
TES: Daggerfall recreated in the Unity engine with tons of improvements (Download the base game from here)
???
Solarus Games
Game engine for making Zelda-like games w/ many completed games available on their website
SRC Available
Flare
Isometric Diablo-like action-RPG
Creative Commons
Dungeon Master CSB
Modern Engine for the 1987 First-person dungeon crawler
???
Adventure
Name
Description
Open Source?
Beneath a Steel Sky
Classic point'n'click with great art and voice talent - Available in most distro's repos
Source Available
ScummVM's Freeware Games
Fantastic point'n'click games here, my favorite being Dreamweb
???
Aye Fair Lady
Point'n'click combined with a musical
No
Heroine's Quest
Excellent Point'n'Click/RPG hybrid inspired by the Quest for Glory series from Sierra
No
Black Sect 2
1st-person dungeon mystery game where you play as a monk investigating a crypt
No
Space Quest II Remake
Excellent VGA remake of SQ2 with voice acting, now available for Linux!
No
Strategy
Name
Description
Open Source?
FreeCiv
Sid Meier's Civilization II Clone
GPL
FreeCol
Sid Meier's Colonization Clone
GPL
UnCiv
Civlilization 5 style empire buillder
MPL 2.0
FreeOrion
Master of Orion Clone
GPL
OpenRA
Modern engine for C&C: Red Alert, Tiberian Sun, and Dune 2000
GPL
Tanks of Freedom
Turn-based strategy, similar to Advance Wars on the GBA
MIT
The Battle For Wesnoth
Turn-based fantasy RPG/Strategy hybrid
GPL
Hedgewars
Worms Clone
GPL
Argentum Age
Fantasy collectable card game
GPL
Wyrmsun
Warcraft II-like RTS
GPL
0 A.D.
Superb 3D RTS, professional grade (Not suitable for weak PC)
GPL
Total Annihilation Prime
Fan-made prequel to the 1999 RTS Total Annihilation
GPL
Zero-K
Polished RTS with multiplayer support, available on Steam
GPL
Warzone 2100
RTS that has campaign, multiplayer & singleplayer skirmish modes
GPL
Thrive
Game about the evolution of life
GPL
Scorched 3D
Turn-based & real-time artillery strategy game
GPL
UFO2000
X-Com-like with Multiplayer functionality
SRC Available
UFO: Alien Invasion
Turn-based tactical combat inspired by X-Com
SRC Available
OpenApoc
Open-source implementation of X-Com 3: Apocalypse
GPLv3
The Fertile Crescent
RTS set during the Bronze Age in the Ancient Near East. Manage a delicate balance between food surplus and the maintenance of your army - Multiplayer mode available
No
TripleA
Turn-based strategy and board game engine inspired by Axis & Allies or Risk
GPL
Star Ruler 2
Turn-based 4X/RTS Strategy space colonization game - This is a commercial game that was open-sourced, you must compile the game from source code to obtain it for free. Consider buying it on Steam or GOG to support the devs
MIT & CC-BY-NC
Beyond All Reason
Open-source remake of Total Annihilation, a Sci-fi RTS
GPL & CC-BY-SA
Side Scroller
Name
Description
Open Source?
Sealed Bite
Game-Jam platformer with nice pixel art
CC-BY-NC-ND
Mr. Rescue
Rescuing people from burning buildings while putting out fires
CC-BY-SA
Sienna
Fun runner/platformer
CC-BY-SA
Super Tux
Super Mario Clone - currently in need of help
GPL
Hurrican
2D Platformer Shooter Inspired by Turrican - Mostly forgotten
SRC Available
Abuse
Commercial 2D shooter turned Public Domain
SRC Available
Frogatto & Friends
Platformer with great music and graphics. Available in Debian/Ubuntu repos
No
Commander Keen 4
New engine for classic DOS-era platformer
GPL
OpenClonk
Commercial 2D Mining game turned freeware, very unique, multiplayer
ISC
Mari0
Mario, but with Portals!
SRC Available
Cave Story
Charming platformer, one of the first big indie hits
No
VVVVVV
retro platformer w/ level editor
SRC Available
Spelunky Classic
Roguelite platformer
No
OpenSurge
Sonic the Hedgehog clone with Level editor
GPL
Lost Constellation
Travel into the frozen woods in a folktale from the world of Night In The Woods
No
Himno
Peaceful 2D platformer game with an infinite number of beautiful procedurally generated maps
No
City Builder/Tycoon
Name
Description
Open Source?
OpenTTD
Clone of Transport Tycoon Deluxe
GPL
Unknown Horizons
Village builder inspired by the Anno series
GPL
Widelands
Settlers clone
GPL
Simutrans
Transportation simulator clone with multiplayer ability
SRC Available
Mindustry
Factory builder / Tower Defense hybrid
GPL
Terra Nil
Relaxing city builder about ecosystem reconstruction
No
Arcane Fortune
Inspired by Civilization, SimCity and Dwarf Fortress, Arcane Fortune is a game of empire building, diplomacy, conquest, construction, and deconstruction. Whether or not you build a utopia, or a hell on earth is entirely up to you
AGPL
MMO
Name
Description
Open Source?
Ryzom
Open-source MMO with a unique world [Free-to-Play]
Famous RL that has both a City-builder/management sim mode, as well as a more traditional RL experience in adventurer mode (you can even visit the ruins of fortresses you've built in the sim mode)
No
Roguebox Adventures
Graphical RL with focus on survival
GPL
Cataclysm: Dark Days Ahead
Unique zombie survival RL that's quite thrilling to play
CC BY-SA
UnReal World
Crazy In-depth survival-sim RL based in iron age Finland
No
Dungeon Crawl Soup
Fairly standard RL to find the Orb of Zot
No
NetHack
Dungeon crawler RL continually updated since 1987
N-GPL
Brogue
RL distilled to fundamentals, terminal-like UI
No?
Tales of Maj'Eyal (ToME)
Very polished graphical-heavy RL/RPG
GPL
ASCII Sector
Excellent RL Demake of Wing Commander Privateer
No
ADOM (Ancient Domains of Mystery)
Extremely polished graphical RL with multiplayer
No
DoomRL
DOOM, but as a Roguelike!
No
Sil & Angband
Tolkien Roguelikes
SRC Available
Pixel Dungeon
RL with a nice pixel-art
GPL
HyperRogue
RL built on a mathematically rigorous hyperbolic surface, with an infinite and space-bending world
GPL
Basingstoke
Uniquely free to Linux users only. A tense roguelike that mixes stealth and arcade action. Explore the smouldering ruins of apocalyptic Basingstoke, UK, a world of extreme peril where reanimated undead and ferocious alien monsters roam!
No?
Space Sim/Trader
Name
Description
Open Source?
Ur-Quan Masters
Updated version of Star Control II. Be sure to get the Voice Pack! (Alternative HD version available here)
SRC Available
Endless Sky
2D space trading/combat game similar to Escape Velocity
GPL
Oolite
Elite remake, huge mod support
SRC Available
Pioneer
Frontier: Elite II Remake
GPL & CC BY-SA
Naev
Space exploration, trade and combat
???
Race Into Space
Re-implementation of 1989's Liftoff!
GPL
Puzzle
Name
Description
Open Source?
FloboPuyo
PuyoPuyo Clone
SRC Available
Frozen Bubble
Puzzle Bobble Clone with level editor & multiplayer
GPL
Duck Marines
Chu Chu Rocket Clone
Multi-License
Pingus
Lemmings Clone
SRC Available
Portable Puzzle Collection
39 Old-school puzzle games
MIT
Blockout2
3D Tetris-like Game
GPL
Enigma
Inspired by Oxyd on the Atari ST and Rock'n'Roll on the Amiga - The object of the game is to find uncover pairs of identically colored Oxyd stones
GPL
Lix
Action-puzzle game inspired by Lemmings with over 700 levels, multiplayer, and a level editor
CC0 Public Domain
Arcade / 2D
Name
Description
Open Source?
AstroMenace
Hardcore 3D space scroll-shooter with spaceship upgrade possibilities
GPL
Leapoundary
Arcade-y Puzzle Shooter
SRC Available
SSVOpenHexagon
Super Hexagon Clone
AFL
LBreakout2
Breakout Clone
Source Available
Witch Blast
Binding of Isaac Clone
GPL
TeeWorlds
Online 2D shooter
CC BY-SA
OpenTyrian
Polished Shmup with excellent visuals and music
SRC Available
Kobo Deluxe
8-Way space shooter
SRC Available
Altitude
2D online plane combat
No
Liero
Real-time Worms
SRC Available
Frets on Fire
Guitar Hero clone
GPL
Eat the Whistle
Sensible Soccer Clone
GPL
NeverBall
Super Monkey Ball Clone
GPL
NeverPutt
Mini-Golf with hot-seat multiplayer
GPL
Shootout Inc
If Hotline Miami and SuperHot had a baby, this would be the result. Also check out Super Hotline
No
Hammer Dongers
4-Player top-down party game
No
C-Dogs SDL
a classic overhead run-and-gun from the DOS era updated for modern systems, supports up to 4 players in co-op and deathmatch modes - Installer Downloads
GPL
Racing
Name
Description
Open Source?
SuperTuxKart
3D Kart Racer, similar to e.g. Mario Kart
Multi-License
Yorg
Top-down racer in the Micro Machine's style
No
Speed Dreams
Realistic racing sim
SRC Available
Max Downforce
F1 style arcade racing game inspired by Pole Position
SRC Available
Sonic Robo Blast 2 Kart
Mario Kart style racer with characters from Sonic the Hedgehog
GPL
Pixel Wheels
Top-down 2D Racer similar to Micro Machines
CC BY-SA
Stealth
Name
Description
Open Source?
The Dark Mod
Incredible Fan-made spin-off of the Thief series using the Doom 3 Engine (Not suitable for weak PC)
GPL, BSD & CC BY-NC-SA
Sandbox
Name
Description
Open Source?
Minetest
Minecraft clone written in C++ instead of Java for better performance. Base game is purposefully lacking in features, instead relying on mods from the community to flesh out the experience
CC BY-SA
TrueCraft
Clean-room implementation of Minecraft 1.7.3
MIT
PyCraft
2D Terraria-like ASCII game played in the terminal
GPLv2
The Powder Toy
Complete physics and material simulaton sandbox in a 2D environment
GPLv3
Surreal
Name
Description
Open Source?
Orchids to Dust
Wandering alone on strange worlds
No
Off-Peak
Searching for a ticket in a head-trippy world
No
Emily is Away
Branching narrative in an online chat
No
CHYRZA
First-person flash-fiction horror story, explore a surreal desert while collecting fragments of a memoir
No
Dagdrom
Weird and colorful platformer thing
No
Naut
Explore mars in a convertible - Short little exploring game
No
Leive-Oma
Walk through the woods with your grandma
No
No Players Online
Short psychological horror
No
Out of The Game
The self claimed 'Inception' of video games
No
Sacramento
Explore a watercolor world
No
David Lynch Teaches Typing
It's exactly what it sounds like
No
A Museum of Dubious Splendors
Explore a storybook from the world of Somewhere. A visual feast for the eyes
No
Vehicle Combat
Name
Description
Open Source?
Warthunder
WWII to Cold War Multiplayer Dogfighter/Tanker/Warship Sim (Not suitable for weak PC) [Free-to-Play]
No
MOBA
Name
Description
Open Source?
Dota 2
Valve's entry into the MOBA genre, with native Linux support [Free-to-Play]
No
MS-DOS
Name
Description
Archive.org
Archive.org has uploaded over 6000 MS-DOS games to their library for future preservation, all fully playable for free right in your browser!