Phoenix: I couldn't say enough about squirrelmail. It's got plugins, it's fully customizable (provided you know some html and php), it's very fast, and it's IMAP. Just the plugin feature alone won me over. Best of all, it's free.
Also, I try to stay away from javascript. Squirrelmail itself is javascript-free. There are some plugins that rely on javascript, but either I didn't install them, or they were minor enough that I didn't care if a few people couldn't access them.
Let me know how it goes!
For those of you interested in taking a look at it, you can find it at
www.squirrelmail.org.