Reddit user 'foamed' posted a categorized list of 166 free-you-can-download-them-right-now games, and I thought "Hey, I know people that like free games..."

Platformers and metroidvanias:

• La Mulana
• Cave Story
• Iji
• Spelunky is a roguelike inspired platformer made by Derek Yu. The Xbox remake heavily expands upon the original game in every single way. The PC version has also a lot of different mods which you can download. If you're going to download the original PC version, then I highly recommend you download the unofficial 1.3 version of the game. It fixes a lot of bugs and it also runs much smoother.
• Eternal Daughter is a metroidvania released by Derek Yu.
• Treasure Adventure Game. You can even add the game to your GOG-catalogue.
• Hero is the first game in the "Hero" series. It's only in black and white and it's inspired by old games from the Commodore 64 and Atari era.
• Hero Core is a minimalistic metroidvania. This is the second game in the "Hero" series and it's a full fledged metroidvania (compared to the first game). It's available on both PC and Mac.
• Hydra Castle Labyrinth is a Japanese metroidvania developed by E. Hashimoto (aka. Buster). Here are some mirrors if the original link doesn't work: http://jayisgames.com/archives/2012/01/hydra_castle_labyrinth.php or http://vsrecommendedgames.wikia.com/wiki/Hydra_Castle_Labyrinth.
• Lyle in Cube Sector
• An Untitled Story
• Within a Deep Forrest
• Knytt
• Knytt Stories
• Poacher is a game created by Yahtzee Croshaw. Yes, that Yahtzee.
• Frogatto & Friends
• L'Abbaye des Morts ZX-Spectrum graphics with Manic Miner/Jet Set Willy gameplay.
• You Have to Win This Game - Retro exploration platformer.
• I Wanna be the Guy
• I Wanna be the Guy Gaiden: Act 1
• suteF
• Arvoesine - Short and hard platfomer which closely resembles Ghosts'n Goblins.
• Phenomenon 32 - A really hard exploration based platformer.
• Celestial Mechanica - Puzzler/Platformer.

Action:

• Black Mesa - Remake of the first Half Life game.
• Warsow - Fast paced multiplayer FPS.
• Quake Live
• OpenArena - Quake III Arena clone.
• CSPromod - Free improved version of CS 1.6.
• Mech Warrior 4
• Grand Theft Auto 1
• Grand Theft Auto 2
• Soldat - 2D Counter Strike with jetpacks.
• Marathon Trilogy - Made by Bungie. It's considered to be the spiritual predecessor of Halo.
• Alien Swarm - Top down multiplayer shooter made by Valve.
• F.E.A.R. Combat - Online multiplayer FPS.
• Hidden & Dangerous
• Wild Metal
• Xonotic - Fast paced multiplayer FPS.
• Red Eclipse - Fast paced multiplayer FPS.
• Wolfenstein: Enemy Territory
• America's Army
• Noitu Love - Action filled adventure game made by Konjak.
• Assault Cube - Fast paced multiplayer FPS.

Arcade:

• N
• Super Crate Box is a fast paced arcade game.
• Probability 0 - Platformer with roguelike elements.
• Tyrian 2000 - Classic Shoot 'em Up. There's also an open source project of the game here: http://code.google.com/p/opentyrian/
• Stealth Bastard - Stealth based platformer.
• Hyper Princess Pitch - Smash TV clone.
• Hydorah - R-Type/Gradius clone.
• Virax
• Verminest - Arcade shoot em up.
• Hurrican - Remake of the old Amiga classic.
• Super Smash Land - Super Smash Brothers demake.
• Shoot First - The Binding of Isaac meets Smash TV.
• Open Liero X - Real time Worms clone.
• Frozen Bubble
• Little Fighter 2 - Great beat 'em up with single player/co-op/battle mode.

Strategy:

• Dwarf Fortress
• Command & Conquer
• Command & Conquer: Red Alert
• Command & Conquer: Tiberian Sun + Expansion
• Open Transport Tycoon Deluxe
• Hedge Wars - Worms clone.
• FreeCiv - Civilization clone.
• FreeCol - Colonization clone.
• 0 A.D. - Age of Empires 2 clone.
• OpenRA - Red Alert Open Source Project.
• The Battle of Wesnoth
• Scorched 3D
• Atomic Tanks - Scorched Earth clone.* FreeOrion - Master of Orion clone.
• Ufo Alien Invasion
• Warzone 2100
• A Nation of Wind
• The Wager
• S.W.I.N.E.
• Advance Strategic Command
• Black Market HD
• Transcendence - A real time space exploration game with some roguelike elements.
• King Arthur's Gold - 2D multiplayer building/action game.

Roguelikes:

• Dungeon Crawl Stone Soup - One of the most popular roguelikes out there.
• Brogue
• ToME v4
• ADOM
• NetHack - The old classic.
• Angband
• DoomRL - Doom roguelike.
• Cataclysm
• Rogue Survivor
• Grand Rogue Auto - GTA roguelike.
• Desktop Dungeons - Supports many different OS, phones, handhelds etc.
• The Slimy Lichmummy
• Mercury
• IVAN
• Castle of the Winds

Roleplaying games:

• System Shock 1 - The old classic. Works on newer systems and has mouse look.
• The Elder Scrolls: Arena - The first game in the Elder Scrolls series.
• The Elder Scrolls: Daggerfall
• Dink Smallwood - Old PC adventure/rpg with a lot of extra fan made stories.
• Magebane 2
• Arkoss
• The Spirit Engine
• The Spirit Engine 2
• Ultima 4
• Ultima: Savage Empire
• Ultima: Worlds of Adventure
• Delver - First person dungeon crawler with roguelike elements.

Adventure:

• Beneath a Steel Sky
• Octodad
• King's Quest 1
• King's Quest 2
• King's Quest 3
• Quest for Glory 2
• Flight of the Amazon Queen - You've got to emulate it through ScummVM.
• Dreamweb - You've got to emulate it through ScummVM.
• Broken Sword 2.5
• A Tale of Two Kingdoms

Driving:

• Trackmania Nations Forever - One of the best free racing games out there.
• Death Rally - Old classic. Cheat, kill and annihilate your way to victory.
• Nitronic Rush
• VDrift
• TORCS
• Racer

Online Multiplayer/MMO's:

• Team Fortress 2
• Tribes: Ascend
• Planetside 2
• Super MNC
• Gotham City Impostors
• Battlefield Heroes
• Age of Empires Online
• Star Wars: The Old Republic
• Dungeons & Dragons Online
• Lord of the Rings Online
• Everquest 2
• DC Universe Online
• World of Tanks
• Realm of the Mad God - MMO with permadeath.
• Bloodline Champions
• League of Legends
• Heroes of Newerth
• Spiral Knights
• Savage 2
• AirMech

Other

• Ao Oni - Horror Exploration.
• Yume Nikki - Horror Exploration.
• .flow - Horror Exploration.
• Ib - Horror Exploration.
• SCP Containment Breach - Horror/survival game based upon the SCP Foundation stories.
• Slender - Based upon the Something Awful creepy pasta.
• Katawa Shoujo - Visual Novel (NSFW). Supports Windows Mac and Linux. Cached mirror to download page: http://webcache.googleusercontent.com/search?q=cache:7FoQ8A8MeVEJ:katawa-shoujo.com/download.php+&cd=1&hl=en&ct=clnk&gl=no&client=firefox-a
• Digital A love Story
• Don't take it personally, babe,
  it just ain't your story - Visual Novel. Spiritual successor of Digital A Love story.
• Frets on Fire - Guitar Hero clone.
• FlightGear - Flight Simulator
• Moonbase Alpha - Aeiou!
• Under the Garden - A sequel named Under the Ocean is in development.
• Which - Short horror game.
• Spring Engine - Many different games made for the Spring Engine.
• Gravity Bone
• Osu! - Osu closely resembles Elite Beat Agents and other rhythm games.
• StepMania - Rhythm game.
• Perspective - First person experimental puzzle game.

That should keep you busy until the Winter Sale.