Greg,
I like Confluence -- http://www.atlassian.com/software/confluence/ Their Notation Guide, at http://confluence.atlassian.com/renderer/notationhelp.action?section=all is good.
CiviCRM is developing their new documentation using it. It's not quite finished, but you can look at what they have -- http://wiki.civicrm.org/confluence/display/CRMDOC/Documentation+Home I've helped with a few sections of the documentation and find Confluence quite easy to use. One thing I really like -- if you're editing a page and go off and do something else, when you come back you'll find that Confluence saved what you were doing for you!
I'm also happy with Drupal books. I guess it's not a real "wiki," but it has the functions I'm looking for. Books give structure/organization to content. You can give edit permissions. I've set "Create new revision" (under "Publishing options") as the default for book pages (you can uncheck it, for example, if you're just fixing typos). There's a user contributed module, diff, that shows the differences between revisions. I like books with Drupal forums, for collaborative writing. You can discuss stuff in the forums and build content as book pages.
If you'd like to see an example, go to http://nccommunities.org/how-to, click the tab "Revisions."; You'll see a list of revisions with who made changes and when. You can select two revisions and click the "Show diff" button to see differences. NCCommunities has a long way to go, but I like the way the Help book is developing, http://nccommunities.org/help.
Judy Hallman
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