Stars is a very nice screensaver designed to display a simple approximation of stars at night, as seen from Earth.
Based on user input, a random set of stars is drawn on the screen, of varying magnitudes and colors. After a certain amount of (random) time the stars brightness is adjusted, making it twinkle or shimmer.
You can adjust the number of stars, framerate, twinkle interval, and redraw interval. There is no upper limit on the number of stars, but at some point your system performance will begin to degrade.