Webmailer

Webmailer is an unoriginally named program that catches mailto links and sends them to your favorite webmail system.

You can set it to open any URL or run any shell script, complete with the parameters lifted from the mailto link. Webmailer comes with links for several popular webmail sites, including Yahoo! Mail, Gmail, Hotmail, and several webmail programs, such as SquirrelMail and Roundcube.