Ever click on a mailto: link, only to scream in frustration as Apple Mail opens yet again? The curse of the webmail user is that there is no way to get around this problem. That is, until now.
Webmailer is an unoriginally named program (actually a preference pane) for Mac OS X that allows you to fix this problem. 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 possible webmail sites, including Yahoo! Mail, Gmail, Hotmail, and several webmail programs, such as SquirrelMail or Roundcube. But you can always add your own configuration.
There is a separate version for OS X v10.3 users, visit the program page for the download link.