Popfile (http://popfile.sourceforge.net/) has some really cool features for mail filtering. Basically, it can provide an IMAP proxy (still in beta) that intercepts your moving mail from folder to folder, and adapts the Bayesian filtering accordingly.
The end result: your incoming mail gets put into folders based on how you've sorted it in the past. This happens for every e-mail, not just the ones that you've classified as spam. In the end, it means that you don't really need filter rules anymore.
We would really like to get this feature into Zimbra Open Source. Given enough time, the folks at our company could probably do it ourselves, but we're busy with other things (like actually running the business). So, this post is a request to see who else is interested, and possibly to raise funds to pay somebody to do it.
This feature was mentioned a while back in passing on another thread, but I think it really deserves a lot more attention than what it got before. I will be sending a similar message to the Popfile developers to see if there's anybody there who would be interested in doing this work.