Acegi is a security framework for Spring. In Mule it provides a simple way of authenticating requests via the Security Manager. There is a simple Mule security provider wrapper that delegates to Acegi, which means you can use any of the Acegi Security Providers such as JAAS, Ldap, Cas (Yale Central Authenication service) and Dao.
See Configuring Security for more information about configuring Acegi with Mule.