XtRadius (Popularity: )
http://xtradius.sourceforge.net/ Freeware server allowing external processes to determine access (as from a database or other source). FAQ and mailing list information ...
Funk Software (Popularity: )
http://www.funk.com/ Creators of Steel Belted Radius. Downloadable demo software, technical support and resources.
Radiator (Popularity: )
http://www.open.com.au/ Flexible, configurable and extensible server from Open System Consultants. Site includes information about other consulting services and products.