I've been playing with VMware's new free "player". It was announced on Dec. 12 but I just discovered it recently.
You need the full version to create virtual machines but there are free virtual machines available that other people have created. The two I've tried are the Browser Appliance and Ubuntu 5.10. (The Browser Appliance is actually a stripped down Ubuntu + Firefox.)
I already have my computer set up to dual boot into either Windows or Ubuntu Linux, but rebooting is a hassle. Now I can work on both Windows and Linux at the same time. This is really useful when you want to create software that runs on both platforms.
Warning: These are big downloads - the player is 35 mb, the Browser Appliance is 258 mb, and Ubuntu 5.10 is 520 mb.
Conclusion - good stuff, check it out.
PS. There's also a beta of the free VMware Server software.