Windows only: If you've waited out Vista and you're in no hurry to jump into testing Windows 7, Nimi is a fun little program for adding swanky visual effects to Windows XP.
Borrowing heavily from the special effects found in the Mac OS X system and the Compiz effects in Linux, Nimi has a lot to add to your desktop. You can implement shadows and transparency, tweak windows to snap to screen edges or maximise when tapped against them. Application windows can also be tweaked with Jello-like wiggles, bouncing, or other real world motions. There are dozens of tweaks included in the program; conveniently, when you select one that will interfere with another, the secondary option is grayed out to prevent conflicts. Nimi ran without hangups or any lag on our test machine, but make sure to turn off the settings you've activated before shutting down Nimi. That's because closing the application without doing so leaves some of the effects hanging around. Nimi is freeware, Windows only.