RADIUS is the acronym of Remote Authentication Dial-In User Service, a couple of client-server protocols defined originally in the 90s and then standardized in RFC 2138 and RFC 2139 by LivingstonInc. The first RFI for RADIUS was originally released by Merit Networks in the early 1990's and responded to by Livingston. See History of RADIUS.
A Radius server is a program which implements the central side of those protocols and generally runs on a central authority server box. The remote side is generally integrated in the operating system of a Network Access Server which works as a client.
LivingstonInc distributed for years a free and open-source implementation of the server which is portable among different Unix systems and provides a few essential features and the whole server side protocols.
Unfortunately, the original Livingston Radius server suffers of a few deficiencies, due in part to RADIUS own characteristics and in part to the precise intent of not compromising the business of a more fine and evolute product of accounting and authorization. Today LivingstonInc is out of business, after being absorbed by Lucent.
Radius server Aradial Top performance radius server for radius billing software solutions
AAA Radius server Radius server for ISP/VOIP/Wireless Billing software
Interlink RADIUS Server High Performance RADIUS Server for Carriers and ISPs
* Cistron Radius The first free Radius