FreeRadius

Website: http://www.freeradius.org/
About: http://www.freeradius.org/about.html
FAQ: http://wiki.freeradius.org/index.php/FAQ
Wikipedia: http://en.wikipedia.org/wiki/FreeRADIUS
FAQ Simultaneous-Use doesn't work: http://wiki.freeradius.org/FAQ#Simultaneous-Use_doesn.27t_work; insert into radgroupcheck values(NULL,"dialup","Simultaneous-Use","==","1");; Uncomment simul_count_query in sql.conf, postgresql.conf, or oraclesql.conf (based on the database system you're accessing) to enable Simultaneous-Use.
Wiki (main page): http://wiki.freeradius.org/Main_Page
Mailing Lists - users, devel, announce: http://www.freeradius.org/list/index.html
Support: http://www.freeradius.org/business/
Press: http://www.freeradius.org/press/

Radiator

Also supported.