looking for a solid/good basic registration/login app/script

hi... i'm looking for a good/solid login/registration script that's in the open source domain. it can have it's own db/table structure. i can always rearchitect... i'd like it to have an admin function, and the ability to verify users via email, etc... anybody have a favorite/good app that they've used/implemented. i've look through lots of scripts, and decided to check here for additional input. thanks -bruce