I’ve been looking at different wiki software lately, looking for ones that are free, hosted, and allows for simultaneous editing. (I love MediaWiki for my own use, but we host it here. I like WikiSpaces as a remote-hosted site, but it has some issues with simultaneous editing.)
In doing this, I found this cool site: WikiMatrix. It compares a large number of wikis and filters the list according to your criteria. Very useful.