After much consternation, I finally figured out how to change stylesheets on a Web page on the fly. As an example, I made this bookmarklet that turns regular Metafilter into Bizzaro Metafilter (don’t click on this link, bookmark it, go to Metafilter, and then select the bookmark).
What else can you do with a stylesheet bookmarklet? Well, you can render sites like Yahoo almost completely in the Wingdings font. I’m sure you’ll be able to find something useful to do with them.
Disclaimer: The above bookmarklets only work on IE 4+ on the PC. I have no idea what they’ll do on the Mac. The Bizzaro MeFi bookmarklet also breaks the scrolly MeFi menu. I could fix it, but it would be kind of a pain.