We use Redmine[1], which has a wiki plus a whole lot more. We use the "issues" feature heavily as well as the wiki feature.

The non-technical staff use the repository browsing a lot - the engineers use SVN/git direct access... the non-technical staff only needs browsing access so Redmine covers that.

In the past, I've used Trac[2] as well. Redmine is heavily influenced by Trac, but at the time I made the choice for our company, Redmine supported multiple project a whole lot better.

In the more distant past, we used Twiki[3], which is wiki-only. It has a lot of functionality and is a nice wiki (I still like its markup best).

Twiki is very nice in that it keeps its pages as text in a repository (RCS when I used it, probably still). One thing that bothers me about Redmine/Trac/MediaWiki is that the pages are in a database so, if you have a problem with your Wiki, you have a very hard time getting your notes out of the wiki to see how you fixed it last time. :-/

A different engineering group at a former employer used MediaWiki[4] which obviously is a very competent wiki as well.

[1] http://www.redmine.org/

[2] http://trac.edgewall.org/

[3] http://twiki.org/

[4] http://www.mediawiki.org/wiki/MediaWiki