Citadel v6.51 is now available. This upgrade provides everything: bugfixes, performance enhancements, and new functionality.
Development right now is being driven by the need to make Citadel work well in high-traffic IMAP environments, so there's been a lot of focus on making that part of the system work faster and more reliably. We've also had a lot of really good input from people evaluating the software and installing it in many different kinds of environments, so the two-way feedback loop is kicking in.
The big new feature in 6.51 is a full text indexing service. Right now you can only access it using a SEARCH command with an IMAP client, but more uses will be available soon (such as in WebCit, obviously).