Please forgive the breach of format. Here is a site I came across a while back that has a big listing of free games.
http://www.freewaregames.net/allgames.asp
Some personal recommendations:
War Thunder
Game Type: Multiplayer combat flight/tank sim:
- Fly a plane or drive a tank in large PVP battles
Platform: Steam
Link to Game:
http://store.steampowered.com/app/236390
Mechwarrior Online
Game Type: PVP Mech game:
- Pilot your mech and work as a team to eliminate the opposing force. Choose your weapons and take aim to destroy vital components on the enemy mechs.
- If you played Hawken and thought, "This game is too easy", try this out.
Platform: Windows
Link to Game:
https://mwomercs.com/landingpad
Black Mesa Source
Game Type: Single player FPS
- It is a modernized remake of Half-Life, using the source engine.
- Damn this game looks good. Unfortunately it does not contain the entirety of Half-Life. It is maybe....85% complete. Still, highly recommended.
Platform: Windows (requires Steam + Source SDK Base 2007 (Also Free))
Link to Game:
http://www.blackmesasource.com/
Path of Exile
Game Type: ARPG (Diablo style)
- Like Diablo? Try this.
- A deep skill and class system allow you to build your character however you want. It looks a little daunting at first but it really isn't as complicated as it looks.
Platform: Steam
Link to Game:
http://store.steampowered.com/app/238960/
Tribes Ascend
Game Type: Team based Multiplayer FPS (Projectile based)
- Did you play Tribes back in the day? Miss it? Well what are you waiting for?
- Choose your class, choose your weapons, grab your jetpack and go capture/defend the flag and/or murder some enemies! This game is all about movement and projectile based weapons. Lead your shots and ski down hills to maintain maximum movement speed and momentium while taking out base defenses and capturing flags. Or just play Arena or Team Deathmatch mode if you want to do some killing.
Platform: Steam
Link to Game:
http://store.steampowered.com/app/17080/
Knytt Stories
Game Type: 2D Platformer
- A neat little platformer. Lots of downloadable games, though the default "Main story" is the best.
- Has a little bit of Metrovania to it since you collect powerups during the game. Provides a nice challenge without being very frustrating. I especially enjoyed the puzzle sections.
Platform: Windows, Nintendo DS (Homebrew)
Link to Game:
http://nifflas.ni2.se/?page=Knytt Stories
Cave Story
Game Type: 2D Platformer/Shooter
- I adore this game. Made entirely by one guy this game has excellent gameplay, great music, and an engaging story.
- Multiple endings depending on your performance and decisions throughout.
- Be sure to download the English translation pack (unless you know Japanese).
Platform: Windows, Steam (not free), Wii (not free)
Link to Game:
http://www.cavestory.org/