I just installed a new plugin for SquirrelMail that allows each user to modify their Sieve server side filtering rules. avelsieve will show the current active filtering rules in a nice graphical interface, instead of using sieveshell.
I found avelsieve pretty easy to use. The only thing that was pretty confusing was that when I first ran it, it did not show the rules that I had installed. I had to enter them from scratch.