Wikipedia
FreeRADIUS is a modular, high performance free RADIUS suite developed and distributed under the GNU General Public License, version 2, and is free for download and use. The FreeRADIUS Suite includes a RADIUS server, a BSD-licensed RADIUS client library, a PAM library, an Apache module, and numerous additional RADIUS related utilities and development libraries.
In most cases, the word "FreeRADIUS" refers to the free open source RADIUS server from this suite.
FreeRADIUS is the most popular open source RADIUS server and the most widely deployed RADIUS server in the world. It supports all common authentication protocols, and the server comes with a PHP-based web user administration tool called dialupadmin. It is the basis for many commercial RADIUS products and services, such as embedded systems, RADIUS appliances that support Network Access Control, and WiMAX. It supplies the AAA needs of many Fortune-500 companies, telcos, and Tier 1 ISPs. It is also widely used in the academic community, including eduroam. The server is fast, feature-rich, modular, and scalable.