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!