This demo video from E3 shows why Scribblenauts has become my most anticipated game of the year:
That isn't from the main game, which features various levels in which you have to summon objects to complete goals. The open nature of Scribblenauts - figuring out the most ridiculous solution to a problem by conjuring anything - is what makes this game so appealing. The video above demonstrates just a tiny portion of the usable objects, but it does show that the selection is diverse.
IGN has a brief description of how the game proper works. Joystiq has a more involved one, detailing a couple of the game's levels.
All of this plus a level editor? Endless fun.