Sharing your bookmarks and photos is all well and good. Discussing stuff on forums can be interesting. Editing Wiki pages is better - coming close to a collective conscious.
But the computer has always been an individualistic tool, which sucks. No matter what I do on a computer, it's always just me. Fundamentally, a computer is used for making things; usually, art, writing, algorithms etc. when I do these things in real life, I can do them with other people, at the same time. This is especially true for music, and note taking, etc. This is much more difficult on a computer - mostly, the only way to so it is to write your bit, save it, send it to someone else somehow, and get them to check it, and send it back. I want to do it in realtime.
Googledocs looks like it could be it, but I'm wary of storing anything on google's computers. Also, it's proprietary software, so it's hard to know what you're really getting.
The other option is online whiteboards, of which there are a few. Inkscape has a whiteboard function, but it's difficult to use so far, and may not be included in the next official build (0.46) (although you can include it anytime you build from source).
Basically, what I'd like to see, is probably something like googledocs running over XMPP, where each user has a locally stored file, which is updated when ever a session is started with both users, and can be updated in realtime, with the messages sent as short edits, not the whole file. Damn that would be cool.
One problem I can see with such a system is that if one user wants to edit something, then send it through another static communications system (like an e-list), you could end up with a few duplicates. That might be solved by somehow creating a unique fingerprint (not based on the file contents obviously, but perhaps the date and time, and the original title?), or simply by allowing users to diff the files, and choose which changes to include...