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. 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.