Ward's Wiki primarily contains information on Design patterns, including Antipatterns and Amelioration patterns. It also contains a history of design patterns. It also contains a discussion on a variety of related areas of OO knowledge, e.g. discussions on Agile Methods.
Actually, it contains discussions of almost everything ever. Japanese strategy manuals, Ad Hominem attacks, Science Fiction authors, Star Trek, Sherlock Holmes anecdotes (see bottom) of page, Mathematics, Ameri-centrism, and many other things.
It provided the inspiration for this course.
In addition, this wiki provided much of the initial motivation for Gang of Four 1995 text that provided the first major published text on design patterns.