If we go for forums: StudIP. Awesome demo I got in a conference lately. Take a look at
http://develop.studip.de/studip/forum.php?view=reset (all german, sorry guys). You can only see half the functionality there though. What I really like is their definable color coding or threads. And the ability to print a whole thread (print preview).
This software actually has quite some good ideas that we should reuse for dotLRN.
