C - as the essential, "underlies everything", systems programming language

Lisp (Scheme)- the strange but incredibly powerful ur-language that great hackers think defines cool

Smalltalk - because this is what OO was meant to be

Erlang (or other Actor language) to understand Actors

Haskell - to understand Monads

Javascript - because its everywhere and essential for scripting the browser

One of Python / Ruby / Perl (but it really doesn't matter which) - so you know what a popular, modern, high-level scripting language with good library support feels like

SQL - so you can talk to relational databases

Prolog - because you still need to blow your mind