Radiator is the RADIUS server. Written from the ground up in Perl, it is the most customizable (and, some say, the most reliable) RADIUS server on the planet, miles ahead of other commercial offerings (yes, it is commercial Open Source, in the sense that you get the source code with your license and it encourages contributions).

Having used it to authenticate pretty much everything since the old analog dial-up days (including ADSL, Wi-Fi - including 802.1X - and GPRS traffic) and having compared it along the way to other commercial offerings, it wins hands down - the only thing it can't do is fix you a cup of coffee.