I've played World of Goo and it's brilliant. It has a simple concept behind the puzzles but the humor and creativity is what really drives the game. I also played Gish one time, it's a platformer where you play a ball of tar. To get through the levels, most being caves, you can stick to walls and ceilings, solidify to crush through obstacles, and liquify to fit through narrow spaces. It's very physics-heavy, and by that I mean that you're mostly a slave to the laws of gravity, momentum, inertia, and other physical forces acting on a ball of tar. This sometimes makes you very hard to control. I personally wasn't very impressed, but then again, I barely ever play platformers.