A quick search on google will land you plenty of results regarding this topic.

A lot of people use Wiki's for their documentation and I wish we could where I work ( I'll get them yet).

Here's one thing that I think many people miss about the whole Wiki idea, and it's something soooo very basic, you're going to laugh.
It's such a basic feature of EVERY Wiki that I think sometimes people forget.

Wiki's are designed to be modified by a group of people, and they are designed to be modified by that group of people OVER TIME.

A Wiki is the perfect agile tool in my opinion, and it can dramatically help in the development of anything (I think).

Consider this extremely simple scenario.

YOU, and I mean YOU, have this really great idea. It's general, it's festering in your brain, and you still need to work out the little bits.

So, you create a wiki. You start off by writing out your basic idea - all the stuff that's been festering for a week now, and then you invite some of your most trusted friends to view it and edit it.

Joe (everyone has a friend named Joe) sees a small part of your idea that he has an idea about himself, so he modifies your Wiki to include his ideas about your idea.

Bam! Now you have an even better and (perhaps) more complete idea.

This can go on for days, weeks or months. Hopefully when it's all "done" ('cause who knows if ideas can ever REALLY be completed), you have this extremely great idea that you and your friends thought up together.

Easy, and you haven't done anything that ALL Wiki's provide out of the box.