I was listening to episode 27 of the Herding code podcast and they mentioned this tool called IE tester, so I decided to give it a try.
The versions supported are 5.5, 6, 7 and 8 beta 2.
The interface is very slick, using the ribbon control. One of the nicer features is that you can open different tabs, each using a different engine and compare the rendering side by side. Each tab using only a portion of the screen.
They have this nice screen cast on the usage: