Tools are important, but don't get too bogged down in finding the magic tool. No tool I've found yet has a "document everything magically using tiny invisible elves" tickbox. :-)
A wiki will work fine. Or Sharepoint. Or Google docs. Or you could use a SVN repository. Hell you could do it with pens, notepaper, and a file cabinets if you really really had to. (I really don't recommend that!)
The big important key is you need to have buy-in throughout the organization. What happens in a lot of shops is they go and spend a bunch of time and money on some fancy solution like Sharepoint, and then everyone uses it religiously for about two weeks, and then people get busy with hitting the latest milestone and that's the last anyone hears about it.
Depending on your organization, field, the type of products your developing, etc., there are a few solutions to that, but one way or another you need to set up a system and use it. Appoint someone the official documentation czar, give them a cluebat, and tell them to hit people in the head everytime they say "oh yeah, I'll finish documenting that next week...". if that's what it takes. :-)
As for tools... I'd recommend Confluence by Atlassian. It's a fine wiki, it's designed to work in an enterprise environment, it has a lot of nifty features, it's customizable, it integrates well with some of the Atlassian's other nifty tools, and is basically a pretty solid product.