I think JSPWiki is one of the greatest pieces of open source software out there anywhere. It might not be well suited to everytone running a small website on a shared host because it requires a Java servlet container for hosting, so demands a slightly higher level of expertise than the PHP junk that's out there.
That said, it drops into place and "just works" for a fast tryout or evaluation. At the same time most of the flexibility needed for large-scale, sysadmin-run deployments is easily available, and its not an "all in one big step" thing either: you can gradually add the finer-grained controls as you need them. Any Java-capable sysadmin will have no difficulty.
Furthermore, there is an active, healthy and, above all, friendly developer community.
I am using JSPWiki to run the "static" content side of this website, with only myself allowed to create, delete and edit pages. (Indeed, only I am allowed to log in!) Its perfect for the task. All I have to do now is hack the templates into something nicer to look at. :-)