I'd been hearing a lot about World of Goo recently. When I saw that a demo of the first chapter was available for download on the PC, I tried it out. Amazingly fun. I had heard elsewhere that the WiiWare version was better (pretty much equivalent, but the Wiimote is a very intuitive controller for the game) and cheaper, so I bought the game and have been playing it quite a bit. Am near the end of Chapter 2 (there are five chapters).
Highly recommended. If you have a Wii, get the WiiWare version.