<p>I've only been able to find two thus far, namely <a href="http://tinyradius.sourceforge.net/" rel="nofollow noreferrer">TinyRadius</a>, which itself discourages production use and <a href="http://www.axlradius.com/" rel="nofollow noreferrer">AXL</a>, which is pay-only.</p>

<p><a href="http://coova.org/wiki/index.php/JRadius" rel="nofollow noreferrer">JRadius</a> seems tied to <a href="http://freeradius.org/" rel="nofollow noreferrer">FreeRADIUS</a>, which isn't a library and will need a lot of cajoling to function like one.</p>

List from
http://freeradius.org/related/opensource.html
(not copying the descriptions because the page says it's copyright
rolleyes
):
Cistron -
http://www.radius.cistron.nl/
GNU Radius -
http://www.gnu.org/software/radius/
FreeRADIUS -
http://freeradius.org/
JRadius -
http://www.coova.org/JRadius
ICRADIUS -
http://www.icradius.org/
OpenRADIUS -
http://www.openradius.net/
XtRADIUS -
http://xtradius.sourceforge.net/
YARD RADIUS -
http://sourceforge.net/projects/yardradius