My mail server stack looks like:
Linode + Postfix + Dovecot (w/mySQL & imap) + RoundCube + SpamAssassin + managesieve.
With this setup, I get more powerful mail filters than Gmail, a better webmail client than most ISPs, and spam controls that I can access directly from my webmail client.
It's not the easiest setup to pull off, though. I'll need to roll out a second one of these before long, and I'll try to write it up next time.
EDIT: Or for $5/month/domain (the same deal I give my clients), I'll give anyone unlimited domain email hosting, unlimited accounts, (practically) unlimited storage.
What are some of the plugins that you find useful?
I also have managesieve installed, and there's a plugin for it too, so with imap, inbound email gets automatically sorted into folders for me, and I can control that from RoundCube. The filtering is actually way more powerful than anything Gmail can do (last time I checked).
Those are the two "must-haves" IMO.
Of course I was also given a mobileme account (which uses a pretty slick desktop-like web UI for mail) but I was shocked that they didn't do like a Google-Apps style rebrand of MobileMe email for their own @apple.com accounts. Pretty sad if you ask me.
Way nicer than SquirrelMail.
there's also this - http://lmgtfy.com/?q=open-source+webmail