Well, I believe you're looking for two things here. The best *WebMail* software I've found is the PHP based SquirrelMail available at SourceForge.

If you want to administer mailboxes and accounts, then you're looking at something entirely different which I have no recommendations for. I still haven't found a web based system management system that fullfills my needs, so I tend to find it faster just to loginto the box and issue a few commands or even use a GUI on the box itself.

- wil