Sticky Mail like app for sending messages to users

Looking for an open source private messaging system to use

[mahoogle]

I've been looking for a private messaging application to integrate with my site. I'm looking for something similar to the Sticky Mail app here at webmasterworld. Preferably it will be a PHP based app.

Thanks for the recommendations.

[dmorison]

Most (if not all) of the popular forum systems feature a private messaging system; so I guess one option is to utilise one of those; just without any forums configured.