Comment on xBack

Ehh... not a bad idea... but you can do its main feature with a single terminal command (make sure it's on a single line):
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background

Having said that, I haven't used it, so I'm not sure how useful the extra features are. Worth $10? Your call.