RADIUS

Remote Authentication Dial In User Service, defined in the following RFCs:

• RFC:3576 IANA Considerations for RADIUS (Remote Authentication Dial In User Service) - assignment of VSAs
• RFC:3575 Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)
• RFC:2869 RADIUS Extensions
• RFC:2866 RADIUS Accounting
• RFC:2865 Remote Authentication Dial In User Service (RADIUS)
• RFC:2548 Microsoft Vendor-specific RADIUS Attributes

...and a cast of thousands of proprietary extensions.

• Google Directory
• Radiator - buy this one, Nothing comes even close.
• RADIUS extensions for PHP
• radstock, a RADIUS analyser