user admin/site admin open source functions..

hey... i'm trying to find an "open source" app that would provide me with user admin/site admin functionality. i've been playing with a few of the open source cms apps to get a feel for how they implement this kind of funtionality. i could rip out what i need, but it might be painful for something like mambo, given it's interconectivity. anybody have pointers to an app that you can use as a user admin/site admin function and tie it into a website/app... thanks -bruce bedouglas@earthlink.net