There are many built-in Windows 7 screensavers (bubbles, mistify, ribbons, aurora). Now you can customize all of the screensaver and make them do what you want.
Windows 7 System Screensavers Tweaker
http://winreview.ru has made a really great tool to tweak your Windows 7 screensaver.
The system screensavers tweaker allows you to modify the bubble radius, number of ribbons, camera focus or the amplitude of the aurora screensaver.
You can for example change the size of the bubbles and make them move a lot faster:
The tweaker is fully working on x32 and x64 systems, but it didn’t work flawlessly on my RC build. It should work on RTM.
More Windows 7 Screensaver
You can download a package of screensaver on our Win7 screensaver page (1 short survey)
And you can also download a bunch of free 3D screensaver here (free)