I've had very good experiences with Frog CMS, though chances are depending on what you're wanting it will only deal with user authorisation/authentication for you. But it's about as light as they come (that I've seen, which, given the multitude of CMSes out there, may not be much at all).
Also see this similar question. (Of those, Textpattern is definitely another good one, but may also be too barebones for you.)
