Also Potato, which me and a few friends have been developing for our own use for a while. It will be released as open source soon.

I will make a separate post once the source is released, but for now anyone that is interested can take a look at a small demo system I set up here: http://potato.dhsdevelopments.com/

The server is written in Common Lisp, client in Clojurescript. Storage backend uses CouchDB, messaging using RabbitMQ and the search uses Solr. More technical details here: http://blog.potato.network/