IETester
A "free browser that allows you to have the rendering and javascript engines of IE8 beta 1, IE7, IE6, and IE5.5 on Vista and XP, as well as the installed IE in the same process."
Microsoft: Internet Explorer Developer Toolbar
Invaluable for troubleshooting web development in IE. Wish I'd known about this before today!
ies4osx
Run IE5, 5.5, 6, and 7 in OS X under X11 emulation. It actually works perfectly and installs in minutes with a few clicks.