Games I can recommend inside your criteria:
Team Fortress 2
I did not say the orange box, the reason I am still listing it is because TF2 on the PC had tons of new weapons, maps, gamemodes, animations, voiceacting.
If this would have been on consoles it would have been DLc and thus about 80bucks on DLC just to get all that content (Medic Pack, Pyro Pack, Heavy Pack, Scout Pack, Sniper vs Spy Pack, Classless Pack, Soldier/Demo WAR! Pack, Engineer Pack each counted as 10dollar's worth
We got this for free, and the Polycount weapons update, a bunch of official maps and a rumored second round of class updates are still coming Free
TF2 today is more like Team Fortress 3
S.T.A.L.K.E.R. Shadow of Chernobyl / Call of Pripyat
Pripyat is more streamlined, Chernobyl is still amazing. just be sure to install STALKER complete 2012 (google it)
They arent for everybody but if you like a good mix between realistic gameplay and fun gunplay play this, atmosphere is everything and the game is pretty unforgiving.
Deus Ex
No need to explain this, avoid invisible wars; its not bad but compared to the original it is awful.
Dragon Age: Origins
also on consoles but the console version is downscaled pretty rough, not just in looks but mainly in the number of enemies on screen.
The PC version has the "true" version of the game, a much much better UI and mods that can work around some of the downsides of the game (I use a autoloot mod for example)
Plants vs Zombies
If you even have a bit of interest in either humor/Tower defense you must play this game. I put about 70hours into it. its that good.
Star Wars: Jedi Knight: Jedi Outcast/Jedi Academy
Fun games, good swordplay for the time, still looks decent and really cheap. and I don't even like Star Wars all that much.
Diablo 2: Lord of Destruction
Crack in Videogame form.
Torchlight
a so called Diablo2 clone, not very long and has some flaws but very moddable and a ton of fun. oh and CHEAP
Things I recommend outside you comfort zone:
Half Life
The Originals, also on consoles but not from this generation and they are cheap.
Warhammer: 40.000: Dawn of War 2
This is a hate it or love it game, it comes close to an RTS but its more like a third person cover shooter with squad command (on lower difficulties ofc) also a good introduction to the RTS genre.
Warhammer: 40.000: Dawn of War: Chaos rising or Company of Heroes
A more traditional RTS, but still quick paced so I think you might like it (if you really dislike RTS at least try Dawn of war 2)
Research and Development - Mod
Mod for Half Life 2: Episode 2. short but sweet and a ton of fun.
Mechwarrior for Crysis - Mod
Again a mod but a kickass game.
Rollercoaster tycoon any of the 3
God/Sim game but it has a very unique charm and they are so cheap you should just try them.
I prefer 1 for nostalgia but 2 and 3 are also great (3 is in 3d)
Audiosurf
Fun reflex/racing like game based on your own music collection, must play imo.
Codename: Gordon
FREE platformer based on half life 2, it was made before HL2 came out and its not that long but hey its free and charming. get it on this page [http://forums.steampowered.com/forums/showthread.php?t=851573]
Grand Theft Auto: Episodes from Liberty City
Okay hear me out. ATM its not worth to buy if you played it before but once the price drops pick it up.
The PC version looks a lot better and you can install a trainer to spawn ramps/teleport etc and that just never gets old; also has quite a few high quality mods (handling, first person view)
any Bethesda RPG
Because you can mod them to get rid of stupid features, bugs, etc