One of my favourite wiki programs is PersPective, an open source, asp.net based wiki with a WYSIWYG editor.
The PersPective software itself is simple and elegant. PersPective stores all it’s data in xml files (another key selection criteria in the early days) utilizing a simple Xml schema. That means that the wiki is easy to backup, replicate and repair when the need arises. As a bonus, Alan Slater the author has always been helpful in extending Perspective In many cool ways. The releases of PersPective are of high quality and compatible, a tribute to Alan as a developer.
The extensibility of Perspective is also worth mentioning. It supports a facility called “Raw Includes” that allows new features to the wiki easily.
If you’re after a good wiki, give it a try: