I've been using wikis for collaborative software development for at least five years. I've typically used TWiki. It's a pretty cool tool, but the set-up is non-trivial, especially for a small project. Also, the last wiki I set up got spammed big time - it was on the Internet rather than a private intranet.
Google recently bought out JotSpot before I discovered it. It sounds like a cool idea - they set it up for you, and I guess it could be configured to be totally private, which would eliminate the spam. I'll be very curious to see what it looks like when it comes back online.
In the meantime, I've discovered Google Docs and Spreadsheets as a collaborative tool. At first I thought, free Word and Excel - who cares? I've already paid The Evil Empire for my software. But when you add the Internet storage and collaboration features, it becomes very cool. A shared document becomes a wiki page!
I see two really nice features with Google documents compared to run-of-the-mill wikis: the formatting and editing is a word processor (implemented in Ajax), not another markup language, and the sharing is on a document-by-document, user-by-user basis. In a small team, it's nice to allow Bob to see the document and Jane to edit it. On another document, they can both edit it. Of course, in a large team, configuring this one-by-one would suck.
Another nice feature is that you can upload documents (from Word, OpenOffice, others), so someone can begin something with a word processor (maybe when s/he is offline) and convert it trivially into a pseudo-wiki page. Sure you can cut and past from Word into a blog or wiki, but I hate the way some characters get mangled in the process.
The Google spreadsheets are nice for collaborative project management. I've been using Voo2do for managing simple task lists. It's nice, but the collaboration options are limited: you can share a password-protected, read-only view, but to allow someone else to edit, as near as I can tell, you have to grant them access to your whole account - not just the one task list you wanted to share.
With Google spreadsheets, all you have to do is create a spreadsheet with the tasks (ala Joel on Software), and share it, either read-only or read-write.
I'm currently working on a small project with two other people, and I've just gotten into this. So far, it's great. More news when it happens.