The idea of multiple desktops was something I had never seen in implementation until I started working with Linux, and I have been trying to find a way to implement this in OS X, without luck (I'm not much of a programmer). Well, the good folks at CodeTek did just that with VirtualDesktop. For those unfamiliar with the idea, it is a method of expanding your desktop real estate without the hassle and expense of a second monitor. The unlicensed demo limits you to two desktops, but licensed versions support up to 100! I use it when I am working in Photoshop, or some other app that hijacks your entire screen with palettes and whatnot, and I still need easy access to other open applications.
I highly recommend taking the time to look around at the info on their site, and giving it a demo period. I've already decided that I can't personnally live without it!
[robg adds: I'm not much of a virtual desktop person (I prefer to have everything visible; not really sure why), but VirtualDesktop is a very well done program. I thought it had been mentioned here before, but it hasn't. They also offer a highly unusual 90 day full money back guarantee, no questions asked, so you could actually buy it before trying it, so to speak!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030423140625302