I’ve been playing around with MediaWiki…it is quite a bit of work! However, for someone new to it, it is a but overwhelming. Trying to find documentation on how to modify skins and such proved a bit daunting…
Trying to get my teeth into it.
WP (WordPress) seems to be a pretty nice piece of work. I find that any Open Source web-ap that is extendible through plug-ins or extensions is great. Things like Mambo, phpcollab, WP are my personal favorites right now. (Although, phpCollab is a bit stale and a bit harder to extend…)
Note: Joomla looks like the new replacement to Mambo.