Looked around for you a bit...
First of all, did you see
this list?
Or looked around at the demos
here?
Some suggestions:
1. You can always mod phpBB, or wait for the next release / try and use the 3.0 beta, it's supposed to be a lot better.
2. One of my favorite forums
HWzone (it's in Hebrew, but you can see the structure) is using
Simple Machine's SMF.
it's not OSS, but has a similar
licence
Edit: MODx cms forums are also SMF (and have a beautiful design)
It's very close to phpBB in looks but with a lot more functionality
I don't know how is the Administration but I can ask around a bit.
3. I've been looking at
Phorum, and it looks nice - really realy simple, and supposed to be very powerful.
This is the forum software that
http://www.opensourcecms.com are using in their forums.
4. There is also
http://www.quicksilverforums.com/ that looks nice, but I didn't look into it so much
BTW -
The big guys (for example -
http://ubuntuforums.org) are using
vBulletin, which is NOT free.