Here's something I did a long time ago, then re-implemented last fall for Java.. It's an updated version of the old "galaxy" screensaver mode from xlockmore, using the Sun "SaverBeans" API.
My version makes the star color change according to its speed, from dark red for slow, to orange, yellow, green, and blue for the fastest. It also makes the star sizes variable so they're not all just single pixels like in the original version. I find it pretty mesmerizing.
I had actually submitted these changes to xlockmore way back in 1997 but somehow they got removed at some point.
I've exported the Eclipse project so you can build it yourself if you want, there are a few settings that affect speed, number of stars, star size range, gravity constant and such, but I never bothered to implement a "settings" dialog so you need to recompile it to play with them. I just fiddled with it until I found something that worked well on my machine and left it alone after that.
If you run the "debug" target in the included ant script it will instead run the saver in a window.
If you just want to run it as a real screensaver (and you trust me), there's a zipfile inside called "galaxy-win32.zip" that contains the necessary files and a readme that explains how to install (basically, copy the 3 files into your c:windowssystem32 folder and check your screensavers)