Simple:
http://www.ffxiah.com/mbox-s
It's interface is inspired by GMail, so Gmail users will be familiar with it. It's something I always wanted to do so I hope you like it. I will just list out some features.
Conversations
BBCode enabled
Auto-Completion of recipient
Usernames/Characters have a separate message box. (no I can't combine them or forward). I did this to make the backend design simpler and other reasons that I can't yet articulate.
Automatic updating, w/manual refresh
Does not mark spam automatically :)
Static version available
Check all button!
I attempted to automatically put your existing messages in conversations based on the subject line and the recipient.
I tested it in Chrome,FireFox,IE8,IE7,Safari, and Opera. From the ones I tested, Opera gave me some problems and Chrome performed the best. I am pretty sure IE6 doesn't work.
I made a simple or "static" version in case the main one doesn't work for you. I put static in quotes because it still uses Javascript but it doesn't use AJAX. I apologize for any bugs you may experience. If you get one you can let me know and I will try to address it.
Keep in mind PM buttons throughout the site won't work until I adapt them.
Images
Folder View
Conversation View