This is not going to be an in-depth analysis, as I haven't yet made any of these tools a regular part of my work, but having had a look I'm quite enthusiastic about all of them in principle. A general (obvious) point: collaboration is only going to work if you have someone using the same tools to collaborate with.
We all badly feel the need (I do, anyway) to get away from torrents of emails, attachments, and clogged-up shared drives. All of these tools could help free us from them.
Google Docs seems easy to use (I just need an opportunity now!) It could function as a shared drive without the clunkiness.
Dropbox could in fact also be useful for personal things - it wouldn't have to be for collaboration. "Always have your stuff, wherever you are", it promises - it's a way to store online with access from multiple computers and no need for USB sticks or, ahem, floppy discs (yes, I know. Technology has been moving a bit too fast for me lately!). I'm never sure how much we can actually trust tools to remain as promised (there was a lot of recent anguish over changes to Delicious) so I'm probably not yet ready to believe the "always" bit.
Wikis are the only one of these tools which we have vaguely discussed using at work. We have a cataloguing manual which really needs to be accessible in a more flexible form (which could be amended easily), and I can see a Wiki being a good potential place for it.
I've been a bit slow to embrace some of the tools available for collaborative work, not least because I have not felt able to warm to our in-house "communities" tool which is designed to do some of this. I do very much see the benefit of getting away from all the duplication of emailing and forwarding documents, so perhaps I should try a bit harder with our own system. Unfortunately it operates through a portal which seems to time out very quickly and frequently. I must be more patient with it and accept the inconvenience - or persuade colleagues to migrate to one of these tools instead!