Ask HN: Any good open-source user management libs for web apps?

I'm building a web site, and want to authenticate/authorize users, do password resets, and possibly link accounts to facebook/twitter.

Anyone make a nice library or web service that does this? My gut feeling is that everyone just rolls their own since it's pretty easy to implement this stuff. But was curious if anyone has implemented such a thing in Java, Ruby, Perl, or Python, or as a web service.