Stop IE [archive.org] is, as its name implies, a negative campaign. It focuses on the security risks inherent in using Internet Explorer and provides a list of alternatives, though Firefox is the only one it deals with in any depth.

Browse Happy is my favorite of the bunch, because it’s an inclusive campaign. It’s run by the Web Standards Project, so the goal isn’t to promote Firefox or eliminate Internet Explorer, it’s to promote choice and get people away from today’s Internet Explorer. The WaSP’s ultimate goal is to encourage people to build a vendor-neutral web in which you can use whatever browser you want—including IE—and get the same high-quality experience. That’s a goal I can agree with, and that’s why Browse Happy is the one I promote. The meat of the site is stories of people who have switched away from IE, with profiles of four browsers: Firefox, Mozilla, Opera, and Safari.

Update (June 2007): Stop IE is long dead. I’ve updated the links to point to the Internet Archive of the site.