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
I'll have to second the vote for DokuWiki, very small very fast, no database dependency Uses smarty so it's skinable(lots of templates already) very extensible.