Almost every screensaver site I have been to in the past has some sort of spyware/adware attached to the files that you download. Really Slick Screensavers is completely different because their downloads only contain a help file and the screensaver file. On top of that their screensavers are not like anything you would find anywhere else.
I played around with a few that they have listed and I think the Skyrocket one (pictured above) is my favorite It is pretty much a fireworks show that is in a 3D world. Oh yeah, and I can’t forget to mention that it has pretty cool sound effects!
The screensavers are all available for Windows but most of them have also been ported over to Mac OS X and Linux. If you are into building your own screensavers then maybe you would like to download the source code for them to see how these are made.