GNU Radius is a RADIUS server and an accompanying set of monitoring utilities. It features MySQL and PostgreSQL interfaces for authentication and/or logging (the ODBC interface allows you to use almost any existing DBMS for that purpose), the ability to rewrite RADIUS requests from various NASs to normalize them to a more understandable format, and the ability to completely customize the behavior of RADIUS authentication and accounting based on NAS and user attributes.
RCDevs OpenOTP Radius Bridge provides a RADIUS RFC-2865 (Remote Authentication Dial-in User Service) API for the OpenOTP Authentication Server. It is an optional server component to be deployed on top of your OpenOTP infrastructure. Radius Bridge is implemented over the powerful FreeRADIUS software. The RADIUS API interfaces with VPN servers and many other software applications requiring end-user authentication. The supported authentication schemes are based on One-Time Passwords technologies (OATH HOTP/TOTP or mOTP Software Tokens, SMS One-Time Passwords, and Mail / Secure Mail One-Time Passwords).