Outside of the typical BI gigs, I do provide some consulting on the capture of non-numeric knowledge.

The products I normally recommend are Fogbugz (if the client is looking for a comprehensive solution that includes case management) and Mediawiki, which is the engine behind Wikipedia.

Sometimes, however, you don't need a heavy duty wiki. For that, I suggest looking into WikkaWiki. It's PHP/MySQL based, and it's pretty easy to set up (although if you're doing URL rewriting, it's a little more involved). I have tried DokuWiki in the past, but WikkaWiki looks great out of the box. DokuWiki still has that "programmer's interface" that so many open source projects have.

Jumping into my high horse... Companies need to remember that their most valuable intellectual property usually rests inside the heads and inboxes of their employees. Wikis, especially those that are open, are essential to any business capturing, retaining and sharing knowledge throughout their organization. Granted, it's tough to create a culture of sharing, but it's an important step to take.

Off the high horse now... Check out WikkaWiki!