One of the cool things about the mac that is different then XP is it’s ability to natively allow you to select different backgrounds for different monitors in a multiple monitor setup. I was surfing the ‘net the other day and came across this resource:

http://www.mandolux.com/

They have quite a few high quality backgrounds with multiple resolutions for both the left and right monitors. That is awesome because I use my MacBook with a 20 inch’r and they both are at different resolutions. It create a really cool effect to have it span across both monitors as one image. They even have an RSS feed to notify you when they add more.