We're using Shrubbery's tac_plus (
http://www.shrubbery.net/tac_plus/) TACACS+ implementation on a linux box to do authentication (against our AD domain via ldap) , command logging, and access restrictions. Just in case that the "no choice" boils down to "feeding money to Cisco"
Tacacs works with all the 15.5.* firmware versions that we have.
Sorry, it's been a while since I touched anything Radius - I'm not sure where to grab a free/GPL/etc implementation anymore