Wikis

I do like the collaborative aspect of wiki's. It's always a problem when working on a project with others to keep track of all the emails, versions of documents, revisions etc. And then when someone else gets involved or you want to share then information with others later it can be a real problem compiling all that data.
The nice thing about Wiki's (and blogs) is that you post it once and then people can edit or comment - and it's all laid out sequentially so you can trace the development or progression of ideas. When others come later it's already there and laid out for them & they too can add their contribution.