Well, the name almost says it all. Open IE in a Firefox tab. For those persistent and ever so incompatible sites. It gives you a little button for your toolbar to reload the current page in IE, then the button changes to a little one that lets you bring it back to firefox. You can middle click to open the site in the other renderer in a new tab.
I realize this has been around for a while, but it's new to me and solved a problem for one of the more important users that I serve. I was using a little bookmarklet to launch IE, but this is far superior. Plugins are a wonderful thing.