At Texas A&M University, we use a simple CAS module for authentication in Rails applications. Note that the script assumes a modified CAS server, so you'll have to make changes if you are running a spec-following version. The documentation in the script explains this issue as well as useage.

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RubyCAS-Client

The library integrates directly with rails 2.3 and 3.x and can also be made to work with Sinatra and Padrino.

Rack-CAS

Rack-CAS is simple Rack middleware to perform CAS client authentication.