What systems you have access to depends on your relationship with the department. All current Computer Science students (graduate students and undergraduate CS majors) and CS alumni have login access to xenon.stanford.edu via ssh. If you are working with a professor in SAIL, you have access to robo.stanford.edu and other SAIL systems. Similarly, if you are working in the Theory Lab you have access to theory.stanford.edu.
Note that each group/lab will usually have just one or two hosts that are guaranteed to allow ssh from anywhere. As of this writing these are:
Other hosts within a group might allow ssh connections temporarily. Do not rely on them.