[eluser]MCrittenden[/eluser]
There are quite a few authentication helpers that CI users have created.
The biggest and probably most widely used is FreakAuthLight (v. 1.1). It does pretty much everything you could ever ask of an authentication helper (i.e., registration, login, forgotten password, adminstration panel, etc.)
A few others include CL_Auth (my favorite..search the forums), ReduxAuth, Sentinel, SimpleLogin, etc. There is a wiki page with a lot of them, and tons of information can be found on all of them by searching the forums.
Hope that helps.