I use GMail and want to have mailto: links be handled by GMail’s web interface, rather than Mail.app. Jack Dorsey wrote a simple handler program called GMailto. It was nice: set it as the default email application, and any email links clicked would call up GMail’s new message composing page.
Then, it stopped working. Thankfully, source code is available, and I was able to make the little mods to get it working again. Here’s the hacked source. You’ll need to compile it yourself with XCode.