What The Hell Does Sharepoint Do?
TwoSheds: MindTouch (http://www.mindtouch.com/) competes directly with SharePoint and the base system is open source. Aaron R. Fulkerson from the company creating it was interviewed in FLOSS weekly recently (http://twit.tv/floss89) Sounded pretty cool, and certainly worth a look if you have any doubts about introducing yet another lock-in to Microsoft platform.