some really good CMS's that i've used on various community related projects include:
DragonflyCMS (Very powerful, secure, updated, and supported)
MemHT (Design supports unlimited amounts of projects)
and a friend of mine developed a barebones CMS that has News, User Registration, Ranks, and file management... im not sure what its called but if you just want a backbone to code on top of without all the features that more powerful CMS's have, i'd search for this one.
i'd say it could be found on sourceforge maybe under the username Mediator...
good luck, and PM me if you need admins / developers!