I consider myself to be something of a programming language enthusiast, so this is a list of programming languages I consider to be valuable to learn. The top three tend to be my languages of choice unless I have a good reason to use something else for a project.

Haskell

Purely functional programming language with completely controlled side effects and a very powerful type system including type inference

Common Lisp

A modern revamp of the LISP language. Comes packaged with a modern object system while keeping all of the LISP magic of old

Ruby

A much more natural modernization of Smalltalk-esque objects than is provided by the more mainstream languages of today

Perl

The go-to language for text processing problems; known for its one-liners

CoffeeScript

A web language that compiles to JavaScript but supports many functional programming features and boasts a much prettier syntax

Io

A very small embedded language that subscribes fully to a prototype-based object system

Python

A popular scripting language that takes C-style classes and makes them sufficiently dynamic

Scala

JVM Language with a heavy emphasis on functional code and delivering a scripting-esque feel in a compiled language

J

An analytical language that aims to keep the APL ideology of incredibly terse yet meaningful code

NB: While I do consider it to be valuable to know mainstream industry languages, they are excluded from this list as they are, from a language theory perspective, not as interesting as the languages listed here.