Out of the box, OS X offers few ways to customize the appearance of the Dock. But lots of great third-party tools have been developed to address that. Unsanity's ClearDock can make the Dock's background transparent or fill it with a color of your choice. After installation, find ClearDock in the Application Enhancer pane in System Preferences. It's offered as a Universal binary, so both types of Mac hardware can play. Owners of PowerPC-based Macs may also want to check out TransparentDock ($8), which offers some features not found in ClearDock.

Poof! Make the Dock's background disappear.