Magicka
From the free demo, it was relatively easy to tell that Magicka is a delightful little co-op magic action game, not unlike Crystal Chronicles or Trine. The magic blending system is straight-up awesome: eight principle elements, with several further elements from particular combinations (such as Fire + Water = Steam), and innumerable magic spells that can result from different amounts and combinations of these elements. I'm incredibly impressed with the depth of the magic system, not to mention its potential for hilarious accidents.
That said, I haven't actually tried the game with multiple players, as no one I know has the game yet. But since it's only $10 - cmon! - I took the plunge and decided to toy around with the Adventure mode a little. And something that the demo really doesn't show, due to its brevity, is the fantastic humorous writing. Magicka's world isn't just a parody of RPGs and Lord of the Rings, but is also chock full of pop culture references, especially in its amazing list of achievements. Magicka goes to great lengths to make sure you aren't taking it too seriously.
Unfortunately, this otherwise fantastic game is slightly diminished by its general performance issues, bugginess, and some gameplay quirks that may or may not be bugs (like enemies that attack too quickly to defend against). But even unfinished, Magicka is worth well more than its paltry $10 asking price.
Progress: Chapter 4: A New Hope