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/