Well I've basically played Fallout 3 and TF2 to death and I am looking for something else to play.  I'm just really not sure what, I've played most of the big name games over the past few years, but since I was not to big into PC gaming before that I'm not sure what games are really worth playing.

Play Half Life 2.

If you've already played it, play it a second time. If you've already played it twice, you're done.

You say you've played big name titles, were these on PC? Are you looking for more niche games or stuff from the past you might have missed or what?

Play some Hinterland. That game is pretty good.

AVP2, No one lives forever 1 and 2, Silent Hill 3, GTA vice city with the vice city deluxe mod, IGI 2, Max Payne 1 and 2, Grim Fandango, GRID, Killing Floor

Left4Dead, Mass Effect, FEAR, Company of Heroes.................. Just to name a few.

Halo: Combat Evolved 

I recommend Mass Effect, but a word of caution... many and I mean MANY people have had problems running this game for whatever reason including crashing to the desktop, not starting, etc. So if you do plan on getting this game just watch out for that (though Bioware has promised a patch out soon), nevertheless it's a great game when it works on a computer. And for $20 on steam, why not? (=

Planescape Torment
Fallout 2
Neverwinter Nights
Diablo 2 <--- word of warning:  this one will steal your life if you let it.
Supreme Commander

warcraft 3 reign of chaos and the frozen throne

Left 4 Dead and get STeam if you dont already have it, its not a game but its a great way of getting them

System Shock 2. The game might  be old but it s still one of the best FPS games.