“MOZiE”:http://www.zeit.ca/mozie/ is a neat little tool that lets you compare the rendition of a web page in Mozilla and IE at the same time. Discrepancies are much easier to spot this way, and I like the scroll-synching! A nice touch.
There is also buttons to switch between a few windows sizes, to check what the page looks like on different screen resolution.
As the picture shows, I have yet to fix the positioning of the navigation bar on the right. On IE it’s nudged inside by about 20px now.