Windows: The built-in virtual machine system in Windows 7 is bare bones. If all you need is to launch XP-mode to run an occasional application, not an issue. XP-More makes managing, creating and copying virtual machines easier.
If all you've done with virtual machines is boot into XP to work around some compatibility issues you may not have noticed what a huge pain it is to duplicate and copy virtual machines created by Windows 7. Rather than get your hands dirty with manually editing files every time you want to make a new virtual environment for testing purposes, use XP-More. It streamlines the process of creating, duplicating, launching and deleting virtual machines. The interface is spartan but it gets the job done and never forces you to manually edit an XML document.
XP-More is open source, Windows only. Have a favourite tool for managing virtual machines Windows 7-based or otherwise? Let's hear about it in the comments.