For me it's been:
Borderlands Claptrap DLC
Everyday Genius - SquareLogic (Hasn't been advertised I don't think but it's 85% off ($1.50 I think), sudoku like game)
FEAR 1
Misadventures of PA Winterbottom (Again not advertised but it is fiddy cents and has a 79 on metacritic, haven't tried it.)
Overlord Bundle
Recettear - The Item Shop
Spore
Titan Quest Bundle (Had the CD's somewhere, but steam is easier. :P)
Tropico 3
Actually thought I bought more so I'm proud of my restraint.
So far my favorite buys have been Recettear and Everyday Genius, Tropico 3 looks pretty promising though. If they do a re-sale today I'm hoping to pick up fallout 3 bundle and new vegas, should have got them first time around.