Multi-version Internet Explorer testing
IETester is a free program which allows you to view Web sites with different versions of the IE rendering engine. Immensely useful if you have a Windows box available:
Of course, WINE has been able to do something similar for quite a while. The IEs 4 Linux bundle works well for Linux and even for MacOS X (with a little effort).
Microsoft has had a set of virtual disk images around for a while that also permit testing, but because it's a full virtual version of Windows, they expire regularly. Which means you have to keep downloading them. Over and over.