VMware Server

The VMware server is an excellent virtualization product. It allows you to run operating systems within a virtual environment - which means you can run different operating systems, or isolate systems for security, trial, or testing purposes. VMware also makes other products, most of which are not free (can be rather expensive in fact, such as ESX).

Xen

(nyt)

VServer

(nyt)

WINE

While Wine Is Not an Emulator, it is used to run Windows software in Linux. Works very well for quite a few things that normally run under Windows.

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.