Cunningham & Cunningham, Inc.: This is apparently the first wiki ever produced — the wiki seed from which all others grew, if you will. It’s also known as the Portland Pattern Repository.
It’s kind of the oracle of all things wiki. There are pages on wiki and Extreme Programming philosophy, like the theories of Wabi Sabi (something all developers would use more of), Once and Only Once, and the Open Closed Principle.
It’s very long on theory and a bit shorter on practical programming techniques, but still very much worth browsing from time to time. Go there for a dose of The Big Picture every once in a while.