The Yale CAS client distribution includes modules for Apache which serve as a CAS 1.0 casclient. See AuthCAS for an alternative implementation of an Apache module for CAS authentication which offers additional features.
Esup-portail also distributes a modified version of MOD_CAS.
A version of MOD_CAS is also distribued as RPM Modules.
We now have a mod_cas patch below for apache2 that contributes support for apache directives for all configuration items. Please note this is not extensively tested at this time. I believe there is still a little bit of work to be done. We plan on testing this here at Athabasca University within the next little while (currently May 27th, 2005). This was written by Trenton D. Adams and anonymous. If someone knows who anonymous is, let me know. I will search my email soon to see if I can find out who it was that helped me.