My wife wants a Snopes.com plugin.
Don't you get tired of people sending you urban legends? It takes a minute or two to recognize that something is an urban legend, find the right page on Snopes.com, and then send an elitist response that includes a link to it. It sure would be nice to have software that could automate this process.
It'd be nice if IronPort and Gmail could add this feature. I don't think it'd be that hard to create a Thunderbird plugin.