Plone

Another open source CMS - Plone, runs ontop of 'Zope'. Currently we have a demo up locally on an X Serve, in the process of getting an external IP & Domain Name for the server.
Check out their intro movie on their web site for an overview of Plone - looks promising, maybe not as flexible/full featured as elgg, but looks nicer, and perhaps simpler for the end-user than elgg (which I find slightly confusing for creating groups within etc...).