Yahoo! Mailer
Redirects mailto: links in your browser to bring up a new message in Yahoo!Mail.
Yahoo! Mailer is a ridiculously simple OS X program that does one and only one thing - it redirects mailto: links in your web browser to bring up a new message in Yahoo!
This means that to use this program, you have to set it up to be your default email reader. Normally, this is done by openning up Apple's Mail program, openning the preferences, and then setting it as the default email reader. Once you've done that, you're ready. Go back to your browser and click on an email link, it should transparently and invisibly open up a new browser window with the compose screen filled in with whoever you're sending to.
You can customize the site you redirect to if you don't want to use Yahoo! Mail.