Open Source Classroom has an excellent in-depth examination of most of the main wiki options that I’ve seen in quite some time. The requirements are a little different from most (collaboration amongst 10 and 11 year old school children), but the differences aren’t really that different. It’s not just children who don’t want to learn complex wiki markup, and many companies wouldn’t really want uncontrollable Google ads throughout their site either.
Mediawiki, Pbwiki, Jotspot, Wikispaces, Dokuwiki, Netcipia, Editme, and Wetpaint are all considered, and all come up short. You can read the review yourself to see why, and discover which one was eventually chosen.