I've already tried Google Docs as part of last years 23Things so for my thoughts on that see here.
Having had a bit more of a chance to play about with it though (mostly for my very small contributions to the CPD23 project) I think it's great. Once you get to grips with the mechanics of it it makes you wonder why you would ever go back to email etc again for organising things. Wiki's work in much the same way although I have less experience in using them. I think that Google Docs is easier for the more techno-phobic amongst us to use since it's not so different from writing normal documents.
I like the idea of Dropbox, but mainly for my own convenience rather than trying to organise anything with others. I'm in the middle of a literature review for my dissertation so I find it really helpful to be able to store documents online. I have a laptop at home and two computers at work so needless to say, the little scraps of notes keep getting lost. Going electronic makes this process much easier (until there's a power cut!)
One word of caution though. Maybe I just haven't played around with the tools enough but there isn't an obvious way to see who has done what and when. Although this lessens the confusion in some ways since you just see the updated document, I think it's useful to be able to see who has made changes and go back to a previous version if necessary.
Overall I like these tools, but more for smaller group projects or personal use than library wide use.