I don't know if this was already on the thread. I ran a search and nothing popped up, so I guess it's okay to post.
Alliance of Valiant Arms is a really fun free, fps shooter. It has a ton of guns which are all modeled pretty well, and animated correctly. Characters look realistic, there is a leveling up system, classes, and TONS of maps. There are a lot of maps, and they are all fun. There are plenty of gamemodes, like tdm, free-for-all, capture the flag, esort, demolition, infection (like Halo) and, scripted missions against A.I. opponents which you can play with your friends. And to top it all off, you can play it on steam. Also, after you play Alliance of Valiant Arms, and then TF2, you get a new butterfly knife for your spy!
The link below takes you to their site, but you can just download it on steam too.