I've been using JSPWiki for years, but haven't really started digging into the innards of JSPWiki until recently.
I believe JSPWiki is a very good platform for collaboration, though the default package is a little relaxed as far as security is concerned. With that said, JSPWiki has given me most of what I've needed so far (support for JAAS and container managed authentication, etc) to work around that.
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