Yeah, this is not very exciting, but it has been driving me crazy for a long time that there was no easy way to do this, since I need email links to click through to IE at work, and want them to go to Firefox at home (and during lunch break). I applied for a sourceforge project for it, but it still needs some finishing touches before I'll feel ready to publish it, like some memory cleanup niceties and a configuration editor and a Readme.
Edit: The sourceforge project was approved and now has a URL: http://browserswitcher.sourceforge.net/
Update again (12/23): Source and installer uploaded!