World of Goo is a popular physics puzzler from 2D Boy that lets you build towers with blobs of goo. Using loosely applied principles of physics, your goal is to move the goo through pipes and into new levels. The game has an enthusiastic following and is available on a variety of platforms including OS X and iOS.
2D Boy is celebrating its 5-year anniversary today and has slashed the prices on World of Goo. World of Goo for the Mac is available for US$1.99, down from $9.99 and World of Goo HD, a universal iOS app, is available for 99-cents, down from $4.99. There is also a non-HD version for the iPhone and iPod touch that is on sale for 99-cents, but as 2D Boy points out, you might as well buy the HD version since it is a universal app and includes both the iPhone and iPad version.