Spot the connection? The thread that ties these three tools together is that they are all used for collaborative working. They allow people to share documents and perhaps more importantly to work on the same document.
Google docs and wikis are both old friends. I haven't used google docs recently but it was a very useful during my Masters course. Using google docs allowed us to work on a single document from our own homes. This made one particular group project much easier.
In the past I have contributed to wikis about teaching and about career development. I've also asked students to create a wiki as a class project. They are an easy to use tool and they can be good fun. As other participants have commented they aren't really something that most people would integrate into their daily working lives but that isn't really their function. Their strength is allowing groups to share their collective knowledge, experiences or learning.
Dropbox on the other hand is a new tool for me. Is it going to become a new friend? Well, I hope so. It looks like it could be very useful. The downside is that I'm not sure I'm going to be able to download it at work. Even if I can't it is still good to know about tools like this. They might prove useful in the future. Also as a college librarian I like to be able to talk to students about the different tools that might help them with their studies.
Tools like these have the power to make collaborative work much easier.