Universe Today
3D Deep Space - this is one of the most popular Solar System screensavers out there. There are several separate versions, and you can buy a pack that contains all their different versions for about $50. There are demo versions of each product, but they've got nag screens and only work for a limited amount of time.
NASA's Genesis Spacecraft - This screensaver comes from NASA and teaches you about the Genesis spacecraft, and shows the path it will take through the Solar System.
SOHO - NASA/ESA's SOHO spacecraft captures images of the Sun. This screensaver lets you display the images on your computer desktop while you're not using it. It works with PC, and they released a Mac version in 2008.
Mars Reconnaissance Orbiter - This screensaver displays images from NASA's Mars Reconnaissance Orbiter
Mars 2004 - This is NASA's site for Mars 2004. Click on Multimedia, and then Screensavers. There are two different screensaver packs you can install.
ESA Scitech - The European Space Agency has its own screensaver software that delivers breaking news from the agency.