This is one of the most refreshingly different games I’ve played in quite some time. The only other game I can think of that is comparable is ‘Portal’, incidentally also available on Steam. Ricochet perhaps comes close, but Goo gets extra points for originality.
World of Goo is a fantastic physics-driven puzzler which will keep you occupied for hours on end, and challenge you with a variety of brain-racking puzzles.
I’ve you have not heard about this game, I strongly recommend you go buy it and if you’ve heard of it, but are wondering whether its worth $20 (yes just $20!), all I can say is what are you waiting for? Its worth every last penny. Get Goo-ing!