Client side script email notification
This will be used on a SharePoint 2003 site with email alerts and server side code disabled.
There is no reliable way to send emails from the client-side in any client browser. The mailto: tag exists, but is only supported in some circumstances and requires cooperation with a local email client which is often not present or not configured properly. With the proliferation of web-based email, there may not even be a locally configured email client.
The only reliable way to send those emails is by giving the email addresses to a server and letting the server send out the emails. If you don't want to do that with your own server or can't, then there are services that can do that for you using client-side forms that submit to their server which then sends the email. One such service is Wufoo. It has both a free and paid version.
Looks like this guy set up a web service and used jQuery to call it to send an email. Not sure if that breaks your constraint or not.