We use mediawiki, and each client gets their own wiki site. I set them up so the client only had read access to the main content, buy has write access to the t

We use mediawiki, and each client gets their own wiki site. I set them up so the client only had read access to the main content, buy has write access to the talk pages. This allows us to publish version controlled specs, with closely tied discussions (on the talk page) for each spec. The biggest problem with this though is getting the clients to edit the talk page correctly.