This runs on Linux as well as Windows, but I find it particularly useful for Linux to virtualize Windows (who would want to waste a box to run windows?! <smirk>).
Innotek VirtualBox is a general-purpose full virtualizer for x86 hardware. Targeted at server, desktop and embedded use, it is now the only professional-quality virtualization solution that is also Open Source Software.
It offers many (not all) of the same feature VMware does, and is worth a look if you want to use open source to accomplish the same thing that closed source VMware does. It even runs on the Mac, allowing you to run windows on a Mac, virtualized <wikipedia screenshot>.