* __[http://newsboard.unclassified.de/|UNB]__ - Only support MySQL as far as I can tell.
* __Build our own__ - {userlink login='drewslater'} did some awesome work on LibertyComments that are inherently a light-weight forum system with nested threads. libertyforums/ would make a fine package but would take a lot of work
* __Tight Integration__ - Tightly integrat any / all of the above options. Lose integration is functional but makes for a messy DB and will always make the BB system feel just not quite right.
+*__[http://beast.caboo.se/|Beast]__ - Lightweight Forum
There is a listing of forum software at [http://foruminsider.com/index.php?act=directory&show=all|The Forum Insider].