Just to throw one more out there: DokuWiki has a lot to be said for it. I'll probably exclusively use MediaWiki in the future, due to my much deeper understanding of the syntax and ability to grab GFDL'd template code and the like from the Wikipedia. In the past, though, I always used DokuWiki for my own sites. It is extremely nice and easy to roll out and, while still full-featured and quite attractive, it has the some of the lowest overhead of any of the mainstream wiki engines. It's plenty small enough to run in an embedded environment, too.

Sean Crago Kathmandu