I love how everyone suggested games you said you'd already played...
de5gravity said:
I really do not care about multiplayer. I like the solo experience in a FPS game, but nowadays it's mostly crap.
I sort of agree, I absolutely love a good singleplayer experience

I know it's not available on Steam but apparently it's freeware now, or at least that's what it said when I downloaded it. But Area 51 was a really fun experience for me, at least 10 hours of gameplay, and voiced by Mulder himself XD
Other decent Solo FPS's:
Oddworld: Stranger's Wrath - I love the Oddworld games so consider me a tad biassed... But it's on Steam for only $10, got great reviews upon release, and really does have some fun interesting mechanics.
Wolfenstein - Surprisingly fun, though not much in the story department
Crysis 2 - written by my favorite author, and well worth a playthrough (or two). It's very fun and succeeds in making you feel like a stealthy badass
AVP - I wouldn't recommend this though unless you're a decent fan of the Alien movies. Because if you are, then the first 3 hours of the human campaign will be absolutely amazing. That's as far as I've played though so I can't comment on the rest of the game.
Serious Sam 1 & 2 for some seriously silly but fun action. Just listen out for... AAAAAAAAAAAAAAAAAAAAAAA!!!!

Oh and in the same vein...
Painkiller - More stupid fun. Watch Yahtzee's review.
Rainbow Six Vegas - These were actually really fun for me. Give the first one a go, the second is more of the same, but loses some of the awesomeness though so perhaps don't buy them both unless you crave more.
Hope that helps! (I highly recommend Stranger and Crysis 2!)
EDIT: OH AND THE UNREAL TOURNAMENT PACK ON STEAM IS LIKE $40! They're all pretty good too.