War for Cybertron does a great job at capturing the fast paced feeling that you'd normally get from watching any Transformers related material (the movies, cartoons, etc). With an interesting storyline and a pretty unique multiplayer, its definitely worth buying, especially during a sale.

Magicka is an extremely fun indie game, similar to Diablo, that is definitely worth the small $10 you'll have to dish out. If you're not convinced, they even have a demo (which you should know is so rare these days), that covers most of the basic gameplay mechanics. With coop, the fun increases exponentially, as you work with your buddies to take out large groups of enemies while combining various spells and attacks.

(While currently filled with many bugs, the developers are releasing patches everyday until all of them are gone.)

Portal is a great title to add to anyone's collection, providing innovative gameplay mechanics, an extremely interesting story (even turning out to be tied into the Half-Life universe!), and top it off, you'll be rewarded with cake. I highly suggest picking this one up, especially with it's sequel right around the corner.