In the past, I've written about Wikis and how useful I've found them to be for collaboration in my company. In the olde days, only geeky types used Wikis. Now, there are a lot of positive changes that make wikis more accessible to the mainstream (you've experience a form of Wiki if you've used Wikipedia, for example).
Here are a few Wiki options I know a lot about, and the links to get there if you really wanna wiki:
TWiki is billed as an "enterprise wiki" and is the Wiki tool that has been in use in my company for the last 6 years or so. It is very flexible and stable, can be customized and is very scalable. However, you must install, administer, and maintain the platform yourself. This is better suited for organizations with a more technical user community and a dedicated (at least part time) Wiki administrator.
pbWiki is a Wiki "platform" including the software and service to get going on a Wiki without installing the software yourself. They have a free version of their Wiki platform geared toward individuals, students, and educators. From their home page, you can sign up for a free account or try out a demo Wiki they've published.
pbWiki also has a small business-centric version of their Wiki platform, with some setup and manageability improvements and more robust permissions management. This costs money, but the price seems reasonable for business collaboration. Here are some videos discussing their new features:
Wiki using Microsoft Sharepoint
Sharepoint has always been a group collaboration tool, but it can be a little (a lot?) clunky to learn. Microsoft has incorporated Wiki features into Sharepoint 2007 to get into this meme. Read more about Sharepoint Wikis here. We're just getting into this in our company, as a way for less technical users to collaborate.
Other options abound
The resources I mentioned are just scratching the surface, and were selected because they are the ones I've been personally exposed to. If you know of other options that you love, please add them into the comments on this post along with your brief review (even a thumbs up/thumbs down or short list of pros and cons).