I have set my employer up with Deki Wiki, which is a FOSS version of their Enterprise Mindtouch Deki software.
This is built as a fork from the same codebase as mediawiki, wikipedia's software. Also, Mindtouch, or Deki or whatever, offers Enterprise features as an upgrade with hosted solutions and various support levels if they find the cost internally is growing out of control.
Also, as someone mentioned above about the power of just doing it, I was able to set it up on old hardware out of my house in order to showcase the feature set to my boss. By doing so she saw that this was something I understood, and could teach someone to manage as I had taught her how to use the majority of the admin features in an hour or so.
I guess it is called Mindtouch Core v10 now.... and they make it a bit harder to get the unlimited core features, with out the trial period sunset, of course. But, stick with MindTouch Core in the selections and you should be fine, provided you have a little know how and a can do attitude.
Good luck! And NO, I do not work for them. I like the free version of their software and found it an easy sell to the suits.