On one hand, screensavers are just utilities - helping you save your monitor from burning out. That was their original idea, anyway, though displays have made quite a progress since those days. On the other hand, it's just another form of entertainment and getting the most out of your PC. Liquid Desktop 3D Screensaver is the ultimate experience of a good screensaver.
Liquid Desktop 3D Screensaver transforms images and textures into melting waves. The default image is your desktop that gets transformed into gorgeous constantly moving waves. There are a few other preset images and textures, but the free version does not allow one to use a custom image. There are also a few options to customize - complexity of the pattern, wave height, and even screen resolution.
The screensaver is amazingly compact, does not require the latest version of DirectX to run (in fact, no DirectX at all is required, which is a plus), and is free. There's the nag window, of course, but that's nothing compared to the beauty of the screensaver.