I've been struck by the sheer beauty of WikiWiki, its simplicity, mild optimism and the way it's one of the few really hypertextual Web sites. A graph, not a tree, at last.
I've written in ASP my own, weak version of WikiWiki, called Weaki. It's mostly in French and still in pre-beta version, but you can take a look at it at http://www.weaki.org