This is by no means a comprehensive list, but rather free games I enjoy or look fun. I purposely left out most MMOs and flash games and wrote this in no particular order.
Let me know if I’ve made any mistakes in spelling, links, or information. I’d also love to update this list with any games you recommend.
Finally, some of these games require other free software, like DOSBox to play. It should be clear on each game’s website if this is necessary. If not, let me know and I’ll put it in the list.
* Ultima 4 - A classic Fantasy RPG.
* Worlds of Ultima: The Savage Empire - A jungle adventure.
* Ultima World of Adventure 2: Martian Dreams - Steampunk space travel.
* Tales of Maj'Eyal - A beautiful fantasy roguelike emphasizing role-play.
* Dink Smallwood - A humourous old PC Zelda-like game. Lots of mods available.
* UnReal World - Low-fantasy roguelike focusing on historical realism.
* System Shock - A classic rebuilt with mouselook.
* Magebane 2 - Help 3 monks find an amulet.
* The Spirit Engine (1/2) - Sidescrolling adventures.
* Barkley, Shut Up and Jam: Gaiden - A tribute to JRPGs & Space Jam.
* Ahriman's Prophecy - An epic inspired by Final Fantasy VI and Chrono Trigger.
* Summoning Wars - A 3D action game with single and multiplayer modes.
* OFF - A surreal and complex French adventure, translated to English.
* Cave Story - The definitive indie game.
* Mari0 - Super Mario Bros. with a portal gun.
* Journey to the Center of Hawkthrone - The game from Community. Made by redditors.
* N - Arcadey ninja action.
* I Wanna be the Guy - Very difficult with free sequels.
* You Have to Win This Game - A metroidvania with a neat visual style. Also on Steam.
* Zineth - Reminiscent of Jet Set Radio. Try and reach the moon.
* Cybermedic - Similar to Zineth, but with more goals.
* Iji - A 2D shooter.
* Within a Deep Forest - Atmospheric sidescroller, but short.
* Maldita Castilla - Similar to Ghosts'n'Goblins.
* Neverball - Similar to Super Monkey Ball.
* Under the Garden - A survival simulator. See the creator's other free games too.
* Street Fighter X Mega Man - Mega Man fights the street fighters.
* In Ruins - An ambient experimental game.
* Dungeon Crawl Stone Soup - Very popular example of the genre.
* Dwarf Fortress - Complex, strategic, and not a roguelike in the usual sense.
* Rogue Survivor - Survive a zombie apocalypse.
* Cataclysm: Dark Days Ahead - Survive in an apocalyptic world.
* DoomRL - Based on the game Doom.
* Castle of the Winds - 2 part game based on Norse myths.
* Mercury - Half roguelike, half arcade game.
* Red Rogue - A gory sidescroller.
* Brogue - Controlled by mouse movements.
* Grand Rogue Auto - Based on the Grand Theft Auto franchise.
* Beneath a Steel Sky - Dystopian point-and-click.
* Transcendence - A top-down space epic Roguelite.
* Treasure Adventure Game - Sail and treasure hunt in 2D. Also available on gog.com.
* Broken Sword 2.5 - Uncover a Templar conspiracy in this pretty point-and-click.
* The Art of Theft - Stealth-based 2D game.
* Naev - Top-down, open world space combat and trading sim.
* 3030 Deathwar - Another 2D space sim available for free or a name-your-price purchase.
* Zelda Classic - Play the original Legend of Zelda, fan games, and make your own.
* Quest for Glory II - Classic Sierra game remade.
* Flight of the Amazon Queen - Point-and-click inspired by movies from the '40s.
* Dragonsphere - Fantasy point-and-click.
* The Wager - A treasure hunting game.
* Heroine's Quest - A newer point-and-click based on the classics.
* Gravity Bone - An artsy, noir, blocky game. Highly acclaimed.
* Deity - An isometric stealth game.
* The Silver Lining - An unofficial sequel to the King's Quest series.
* 0 A.D. - An Age of Empires 2 clone.
* FreeCiv - A Civilization clone.
* FreeCol - A Colonization clone.
* FreeOrion - A Master of Orion clone.
* Battle for Wesnoth - Turn-based fantasy like in Fire Emblem, with lots of additional fan material.
* Open Transport Tycoon Deluxe - A Transport tycoon clone.
* Unknown Horizons - A city builder similar to ANNO..
* Black Market HD - A space trading game.
* Atomic Tanks - A Scorched Earth clone.
* Scorched 3D - Scorched Earth in 3D.
* Command & Conquer (Tiberian Dawn/Red Alert/Tiberian Sun) - The early C&C games.
* A Nation of Wind - A pixelated game similar to Populous.
* Reprisal - Another homage to Populous, expanding tribes in a small world.
* UFO Alien Invasion - A game similar to X-Com.
* 1821: The Struggle for Freedom - Play through the Greek Revolution.
* Nitronic Rush - A bright and flashy racer.
* Rigs of Rods - Driving simulator with soft body physics.
* Rule the Rail! - Build railroads and then ride them.
* MTBFreeride - A dirt bike simulator.
* Flightgear - High quality flight simulator.
* Armagetron Advanced - A Tron lightcycle game.
* Trackmania Nations Forever - Drive on walls and on other crazy tracks. More content is available by purchase.
* VDrift - Concentrates on drifting racing cars.
* Racer - A car simulator.
* Torcs - A regular racing game.
* Death Rally - Blow up opponents in this classic remade.
* X-Moto - A 2D motocross game.
* Rise of Flight - A WWI flight simulator. More content is available by purchase.
* Black Mesa - A remake of the first Half-Life.
* Noitu Love - A quick-paced sidescroller.
* Hidden and Dangerous - A WWII tactical shooter.
* The Marathon Trilogy - The precursors to Halo.
* Tyrian 2000 - A classic arcade shoot-em-up.
* Hyper Princess Pitch - Similar to Smash TV.
* Super Smash Land - A Super Smash Bros. demake.
* Black Shades - Play as a quick-thinking bodyguard. 3D.
* Little Fighter 2 - A beat-em-up that allows four players on one computer.
* Super Crate Box - A simple, but fun shoot-em-up.
* Diaspora: Shattered Armistice - A Battlestar Galatica dogfighter game.
* Wing Commander Saga - A fan-made Wing Commander game.
* Full Spectrum Warrior - A modern military tactical shooter.
* A Fistful of Cows - A western hack-and-slash.
* Slender: The Eight Pages - The original slenderman game.
* SCP - Containment Breach - Survival game based on the SCP mythos.
* Ao Oni - A Japanese puzzle/adventure game.
* Ib - Explore a museum as a little girl.
* Which - The ending changes depending on your choices.
* Hide - An atmospheric pixelated game.
* 1916 - Der Unbekannte Krieg - Survive the trenches of WWI with a twist.
* Ascension - A psychological horror sidescroller. Check out the creator’s other games too.
* ATTACK_ON_TITAN - Hack and slash at giants in 3D.
* Super Mario Bros. Crossover - Super Mario Bros. with different classic characters.
* The Republica Times - The precursor to Papers Please.
* SuperHOT - A FPS where time only moves when you do.
* Fallen London - A text-based RPG with Lovecraftian elements.
* World Golf Tour - A golf simulator.
* Card Hunter - A less complex tabletop RPG.
* Team Fortress 2 - A hat collecting FPS.
* Spiral Knights - Similar to The Legend of Zelda: Four Swords.
* Vindictus - A hack-and-slash dungeon crawler.
* Heroes of Newerth - A MOBA in the same universe as the Savage games.
* Path of Exile - A spiritual successor to Diablo II.
* Warframe - A futuristic third person shooter.
* Quake Live - The everlasting online FPS.
* Tecmo Bowl - The old Tecmo Super Bowl with updated rosters.
* Neptune's Pride - A space 4x game you can take your time with.
* Tagpro - Capture the flag in 2D.
* Moonbase Alpha - Explore the moon with John Madden.
* Hearthstone - A World of Warcraft card game similar to Magic: The Gathering.
* SMITE - The godly alternative to DotA and LoL. Great small community.
* Space Station 13 - Become a member of a space station and keep it running.
* Myst Online: Uru Live - Explore the worlds of the Myst mythos.
* Planeshift - A truly free fantasy MMORPG.
* Planetside 2 - A sci-fi FPS similar to Battlefield.
* Defiance - A MMO that’s also a tv show.
* Unturned - A 3D zombie survival game.
* Happy Wars - A tower defense-like game.
* War Thunder - A WWII vehicle combat game.
* Loadout - An over-the-top shooter.
* Heroes and Generals - A WWI FPS/RTS.
* Neverwinter - Another Dungeons & Dragons RPG.
* Octodad - A 3D QWOP, in which you control a normal, inconspicuous father.
* Space Engine - Fly through space in incredible detail.
* The Powder Toy - A 2D science sandbox.
* Passage - A short 2D game about life.
* Starmade - Like Minecraft, but in space.
* Katawa Shuojo - A visual novel made by 4chan. NSFW content can be disabled.
* Hedgewars - A Worms clone.
* Frets on Fire - A Guitar Hero clone.
* Perspective - A platformer and FPS, where you control the perspective of the level.
* Toribash - A 3D fighting game in which you create your moves. Also on Steam and has online multiplayer.
* Fish Fillets - A puzzle game.
* Broken Dimensions - Walk on walls as a boy in a chicken suit following a ghost.
* Digital A Love Story - A computer mystery romance, similar to a visual novel.
* Don't Take it Personally Babe, It Just Ain't Your Story - A visual novel and spiritual successor to Digital A Love Story.
* Masq - A NSFW visual novel with huge variation of choice.
* Realistic Summer Sports - Play summer sports with QWOP-like reflexes.
* Slave of God - Hang out in a seizure-inducing club.
* Peggle Extreme - A mix of pinball and pachinko.
* Egoboo - A 3D dungeon crawler/hack-and-slash/RPG.
* Façade - An interactive drama.
* OSU - A rhythm game, with lots of Japanese pop.
OTHER USEFUL SITES
* TheFreeBundle.com currently has 12 collections of free indie games. Some overlap with this list exists.
* The Spring Engine has led to the creation of some nice RTS games.
* Kongregate.com has great flash games.
* MMOHuts.com has a list of virtually every free MMO you could ever want, along with reviews.
* PCGamer regularly finds free games to play online or on your desktop. Some overlap with this list may exist.
* FreePCGamers.com has pages upon pages of free games and their download links. Some overlap with this list probably exists.