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.
Vircom (Popularity: )
http://www.vircom.com/ Makers of VOP Radius server. Site includes key benefits and customer comments.
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 ...