I ran a Citadel a couple of times; Waffle was my favorite to use because it had a unix-like feel to it on my PC and I was able to do internet email and Usenet stuff using the UUCP thing but Citadel had my heart too because I loved the rooms concept.
I ran some other experimental ones; FREE SPEECH was an anonymous messaging board; no login or anything. Just get on, post, leave. That was very popular