I came across World of Goo this week, because it was featured in an article on Slashdot. For its1 year anniversary, the makers of the game decided to sell it for a limited time, at the price of your choice. It's basically donation-ware. This proved so successful that they've extended the "pay what you want" period till October 25th... and I highly recommend you get this game. I paid 5$. In World of Goo, you get a certain number of goo balls in each level, and you must build a goo structure, by linking the balls together, to reach the pipe that indicates the end of the level. You'll be crossing chasms, climbing walls, and all sorts of bizarre variations on the theme. It's physics based, super easy to learn, and very addictive. Oh, and it's available for PC, Mac, Linux and Wii!