http://store.steampowered.com/
HUGE Holiday sale going on right now, it last until the 3rd so if you plan on buying games wait for the deals of the last day (and keep an eye on them) and buy what you want at crazy disounts.
My Suggestions from this sale:
"Shooter Pack" 5 really dumb but oh so fun shooters that are made to break the game with overpowered guns.
"Valve complete Pack" (chances of this going on sale are pretty damn big) if you get this thank us all later.
"Rome: Total War: Gold" get a UK based person to gift it, its less than 1.5 pound.
"Torchlight" - Best diablo Clone made by a lot of the makers of Diablo1, has steam cloud support for extra awesomeness.
"Company of Heroes: Gold" Best WW2 RTS there is and one of the best RTS campaigns I have played, you could get Tales of Valor in the deal today but it is mostly a multiplayer Expansion
"Team Fortress 2" It is in the valve pack but deserves a mention
"Mass Effect" I think this will be a daily deal soon, amazing game, if you skip all the optional stuff you do not like it is also very cinematic
"Fallout 3" Not a worthy successor to Fallout 2 (as in a true sequel, *flamesuit ready*) but a great game well worth playing, It was mis priced for 17.50 earlier this week so it might be a deal for that price later.
"Plants vs Zombies" This together with defense Grid and Immortal defense is one of the best Tower Defense games ever, and it has great humor; worth it for the almanic alone.