I had an app which displays fine in IE8, but IE7 is messing up the CSS for some reason.
To debug this, at first I thought i had to use a copy of IE7, which is kind of pain. However, it turns out you can tell IE8 to render in IE7 mode — which in my case succesfully reproduced the problem.
Press F12 to open up IE8 developer tools (Is there a menu option for this? I am not good at remembering secret F-keys). Then simply choose “Browser Mode: IE7” from the menu bar in the developer tools window.
This can apparently be used in IE9 too, to render in either IE7 or IE8 mode.
You can’t go all the way back to IE6 with this method. But I think many of us (fortunately including my shop) have stopped supporting IE6.