Yesterday, I installed the latest version (5RC2) of the open source (YPL) groupware suite Zimbra. After messing with the /etc/hosts file (zimbra is not a fan of IPv6 entries) and tricking it into thinking my Ubuntu 7.10 was 6.10 (modifying /etc/lsb-release and /etc/debian-version) it installed quite easily. After getting it installed, I figured out that I had done it wrong, and I was unable to send/receive emails (my MX records were incorrect). Asside from that, I love it, it's feature-filled, easy to use, and very simple to manage through the web administration console. I like how it has a built in Jabber/Oscar/MSN client, and has a shared documents and calendar system, making it very easy to keep group projects organized. Overall, it's a very nice system, and after some more testing on a beefier machine (one of the blades) I hope to show it off to OIT as an alternative to Exchange, which is very miserable to use.
Peace and chow,