At work we recently set one up which we are planning to use for a variety of internal things, and the person evaluating it settled on [link|http://twiki.sourceforge.net/|Twiki]. Fairly easy to set up. Works well. Lots of plugins in case you want to add features. Plenty of people are using it.

There may be better solutions out there. But it is certainly good enough for what we want.

Cheers,
Ben