We currently use Account Manager Lite for a medical laboratory company that must restrict access to licensed physicians. Because
of our growing base of users, we will soon purchase the full version
of Account Manager.
Account Manager Lite allows restricted access to a certain directory
on your server, and allows potential visitors to register for a
username and password of their choice.
Once a user has requested access, you have the option of allowing or
denying their access via a web-enabled interface. Results of your
decision are sent to the visitor with a personalized e-mail note
of your template design.
Once you have "approved" a user, their username/password combination becomes active instantly without any further programming.
Account Manager uses the common .htaccess method of directory permissions/access.
Ease of installation is moderate.
I would recommend AM Lite for those without a large group of users.
It's free, relatively easy to install and use, and the precursor to
the full version that includes many more search/information features.