World of Goo - PC game
This is an addictive puzzle game where you need to build things with the help of the "goo" -- these cute, oil drop creatures that can be stretched and connected in various ways. This is kind of like a cross between building with blocks and playing Jenga with cute animation thrown in. I both got frustrated and laughed out loud while playing this game. There are 47 levels and it is often fun to go back to a level and try to beat your previous score, or compete for the highest score on the master scoreboard on their website.