innotek VirtualBox is a very nice virtual machine environment. It’s mostly limited to Windows (all the way back to DOS) and Unix/Linux, but it’s easy to use and works well (not to mention free). They say they’re working on Mac support.
This could be a great solution for someone who absolutely must run that Windows 98 (or DOS) application that won’t run under XP.
More about VirtualBox at: http://www.virtualbox.org/