/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &
This tip was mentioned by Ken Dyke at last night's BANG meeting.
[Editor's note: On my G4/733 with the GeForce3, this is simply amazing. The new "flurry" screensaver is running right now on the destop at 1600x1200 in thousands, iTunes is playing, the ink recognition floater is open, and yet the CPU utilization is averaging at or below 50% of thereabouts. Everything is smooth, including the iTunes playback, typing in this text box, and the screensaver. A very cool trick to show your friends the power of OS X if you have the hardware for it. I had closed and opened a new terminal window, so I had to use "ps -ax | grep ScreenSaver" and then "kill" the process ID number to end the effect.]