I have not much more to add on this topic. Google Docs is useful to share the editing of documents, although I do wonder what happens if two people are editing the same document.
My experience with wikis is that they tend to be used as cheap websites. Even though all members can theoretically update content, in practice one person does most of the work.
I cannot use Dropbox at work as it requires installation, and I don’t want to trouble IT. I do have it at home. My wife plays the piano at our church and we use PowerPoint for services. It is a useful way to transfer the Powerpoint from home PC to church laptop, although it annoyingly tells me on startup about changes to files. Most of the time email is adequate for sharing files.