WSAD

World of GOO has been around for a bit, its a wonderful game. Developed by 2D Boy, world of Goo is a puzzle building physics game. It has a whiff of lemmings about it as you attempt to get your quirky, funny little balls of goo from A to B. I installed it on both my sons computers, they are 5 and 7 years old. I can see their little brains grow as the doors to problem solving skills open up before my eyes. They sit and ponder their next move giggling and mimicking the goo's funny language. Its easy to learn to play, and hard enough to keep you engaged, you will laugh, and bang your fist on the desk a little as well! The graphics are great and the Physics of the game are amazing.
You can get it off steam, or go to the World of Goo site- buy it and support these guys!