Forum and Knowledge Base recommendations

First off - no, I'm not looking for where to find these - I know about
hotscripts, etc. - what I would be interested in is your opinions
and/or experiences of what has (or has not) worked for you. I have a
few options, but I'm looking for some input: Specifically, I'm
interested in knowing what you have used in a high-usage environment
(multiple thousands of users, 1,000,000+ hits/month) and what solutions
you have found to be especially stable or particularly unstable.

This would be for separate solutions: I need a discussion forum
(PHPbb/snitz type) AND an searchable knowledge base (perhaps like the
OSCommerce knowledge base).