Sanctum 1 & 2 are FPS/Tower Defense mixes. Gotham City Imposters is amazing. However, The loading issues are a big problem. It is free though. Battlefield 2 is pretty darn awesome, But.. Battlefield Bad Company is a timeless gem. However, Its a console only game. But if you own as PS3 or 360 it is worth it.
Windows 8 - AMD Dual-Core E-3000 - 4 GB RAM - ATi Radeon HD 6310 I will change this sig when Gabe posts a picture of himself eating a Twinkie.
There's a million great fps games. Stalker was already mentioned, which is an amazing set of games. Cryostasis is also great but longer available via DD.
Hard Reset is great.
Personally atm I'm playing some older fps games that I originally missed on the XBOX. Playing Red Faction one of the orignal Ghost Recons right now. I also recommend Chrome. Recently purchased Project Snowblind and Republic Commando.
Also recommend Call of Juarez: Gunslinger and Bound in Blood. Oddworld: Stranger's Wrath is excellent. I enjoyed Bioshock Infinite but I wouldn't recommend Metro: Last Light.