Someone who believes it's their duty to attempt to correct any grammar and/or spelling mistakes they observe. Usually found hanging around Irc chatrooms hounding "n00bs".
Yes, there are plenty of grammar Nazis on the Web, commenting about grammar errors instead of participating in discussions. What I propose is a Firefox extension that grammar Nazis can install that allows them to fix errors on any web page. The other users would see the corrected text.
There could be an option to see what was modified, the corrections would appear as little notes instead of seeing the proper spelling.
Even a Wordpress plug-in
Even better would be a Wordpress plug-in to go with it. When someone fixes an error on a blog with the plug-in installed, the administrator could receive an e-mail informing him that a spelling error was found.
I'd do it if I had the time, I might someday but I'm sure there are people who are more motivated than me to make this a reality.