I use both IE and Mozilla Firebird for browsing. I like to check if my site/web apps I develop looks good and works as intended in both of these browsers. So I built application which has both IE and Mozilla embedded in it. Now I can check out how any site looks in both browsers instantly. The screenshot show Yahoo rendered by both IE (on top) and Mozilla (below). Previously I never noticed they were rendered differently.
I am looking to add more features in it like synchronized browsing and scrolling, integration of bookmarks and favorites. I can put it up for download if anyone is interested.
Update: New version is out. Check out: http://www.vasanth.in/software/comparator/