Mindmeld seems to have a rather narrow focus, but the description sounds nice (and it's free software):

"Mindmeld is an enterprise-capable knowledge-sharing system. It is an effective tool for any web community that needs to capture and share information, and is unique in that the knowledgebase grows smarter every time it's used. It only knows what you teach it, but it's very easy to teach.

Mindmeld's core search technology, the self-organizing neural mapper, allows Mindmeld to incorporate terms used in each search into a contextual map of the answer itself, continually improving Mindmeld's ability to derive contextual information from a given search. The system learns how people typically search for a given answer by identifying which search terms are most valuable in any specific context."