Citadel is absolutely awesome. It takes minutes to install. They even have a virtual machine you can download from their site, built on CentOS, that you can try. I have been using it for a few months now and have had a great experience. It has shared calendars, email, bulletin boards, and more.. and a nice web interface to all of the above (a text only client is even available). All of the features are available in an open source package. There is no seperate "enterprise" version with features that the "open source" version doesn't get. I am running it in a virtual machine based on Ubuntu 7.10 server, and currently testing it in a virtual machine based on Ubuntu "jeos" (Just Enough Operating System). So far it has been great. The "community" has been great about answering questions that have come up. I highly recommend it. It has greatly simplified my job with regards to managing my mail server and calendars.