I've been playing Star Wars Republic Commando, not for everyone for sure but I find it an enjoyable FPS with squad based elements.  Single player is fun!
I keep all versions of Quake loaded for a quick fragfest, the first two look dated, because they are, but they are still a bit of a challenge.
Also, picked up Contract J.A.C.K. for a buck at one of the local dollar stores, I like it. 
And don't forget Armed and Dangerous and both Serious Sam games.  Serious Sam Gold includes 1 and 2 and can be had at EB for about 5 bux.  These games add some much needed humor and smart-ass comments during gameplay, A&D has a Monty Python flavor to it!  

Enjoy!
ARCADIAC!