Assembly language and microcode. A CS student should understand all the foundation layers of abstraction between boolean state machines and the latest high-level functional/OOP/etc. programming language paradigm.
Basic or Logo or Squeak (et.al.), if they ever want to be able to understand how to teach small kids some computer literacy.
Fortran, Cobol and Lisp, if they want to understand the history of programming languages, and what the actual problems are that modern practices fix.