pearljammer on 13 January 2011
Flower and PixelJunk Monsters are my two favourite downloadable console games. Seeing as you already own Flower, I'd recommend PJM.
It's incredibly intuitive, oozes charm, will proved a unique experience within the genre, has co-op and is among the most fun I'h had with any game the past few years.
Edit: As okr mentioned, Monkey Island as well. If you haven't played them before, I'd recommend both before Pixeljunk Monsters.
