Everyone knows the answer if you ask for an example of a successful wiki: Wikipedia. The obvious next question is: what's another example?
Perhaps its just me, but I don't remember hearing an answer to that question yet. In fact, as obvious as the question is, it didn't really occur to me until today. (Not in so many words, at least.)
I can give plenty of examples of wikis and some still active. But successful? Not really.
I know plenty of corporations which are trying to build a Wikipedia inside the firewall. Again, success is elusive.
So, is there really only one Wiki in the entire world?