I'm discovering how really incredible git is for a wiki back-end. Fact is, revisions are great for backups and historical review, but they aren't used nearly as often as the most current version, and if they aren't stored efficiently they can eat up a ton of space before you know it.
I use ikiwiki for public wikis and some general publishing. Its cool because it can use a variety of back-ends: git, subversion, among others, but surprisingly it doesn't even need a reversion control system. Neither does Nesta, but that's primarily for blogs. I only mention it in this post because its written in Ruby and uses Sinatra.
Which brings me to Git Wiki, the wiki I'm most familiar with besides Private Wiki. I don't use it publicly, because it has no built-in access controls, but thanks to EncFS and git, I have experimented with an encrypted storage systems.