Campus Crusade for Christ distributes a CAS server modified to implement a Single Sign Out feature.
We currently use CAS as the authentication mechanism for Concur, a third-party expense reporting solution. We also hope to provide CAS interoperability with RightNow, which we use for incident management. Both of these systems run off-site, so the CAS authentication model was a great fit.
We plan to soon finish our initial "single sign-on" rollout, deploying CAS clients to a number of major internal websites. These sites run on IIS, Apache, and Tomcat.
For the back-end authentication, we use two LDAP servers (one Active Directory and one eDirectory) as well as a PeopleSoft application server (via a PeopleSoft component interface).
Our CAS server is currently running in Tomcat on RedHat Linux on a 3GHz machine with 3GB of RAM (we like having room to grow). We have a second machine which we hope to cluster with the current server to minimize downtime. Most of the work has been completed on a JGroups-based clustering system for the ticket caches to allow this live clustering to work.