http://httpd.apache.org/docs/howto/auth.html is the official document on apache authentication.
You have a number of choices of user databases for authenticating users. You can authenticate users using, mysql, ldap, local users...
The problem is finding an exhaustive list of these modules.
http://theoryx5.uwinnipeg.ca/guide/s...h_modules.html
has a short list, but not the homepages for these modules. I would guess you need to find what authentication you'd like to use first. then find the module from the auth module list or on google, then install that module.
*WARNING!!* Do *not* use an apache module to authenticate users using they system passwords over a normal HTTP connection. That *might* be ok if you're using mod_ssl and HTTPS to encrypt the login.
As a scenario, we use auth_ldap
http://www.rudedog.org/auth_ldap/ and mod_ssl
http://www.modssl.org/ to authenticate users login to private sites and also use these same LDAP servers with PAM ldap and nss_ldap to authenticate system users. We also take further precaution by restricting by IP address using the httpd.conf settings. The apache module allows access to restrict users by groups and other arbitrary ldap attributes.