FreeRADIUS is an open source RADIUS server implementation being used by a number of ISPs and others.
It's homepage can be found at 
http://www.freeradius.org
BlairHarrison has written some FreeRADIUSNotes with respect to setting up FreeRADIUS for use with Telecom's Jetstream/ADSL service in NZ.
One page links to FreeRADIUS:
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