It seems to be one of the most capable, if not the most capable, of the WakkaWiki forks. It can do files, comments, and cluster (see more in the docs). It's got a nice not-quite-WYSIWIG interface and can be integrated very elegantly into a web page. It's also really easy to install (even though the docs don't say that you have to create the config file in the main directory before the installation -- they just tell you to chmod it!). I had it up and running in like 5 minutes.

OMG. I spent the last few days searching for a Wiki engine to use for my Master Thesis. After looking at PmWiki, MediaWiki, TWiki, Trac, PhpWiki, DokuWiki and other engines, I must say this one seems the way to go. It comes with everything I need OutOfTheBox. I've used PmWiki before but this one seems great.