Relevant links:

http://en.wikipedia.org/wiki/Categorical_list_of_programming...

http://en.wikipedia.org/wiki/Programming_paradigm

I'd say learn Haskell, Clojure, and Scala before anything else because they seem like they might actually be 3 of the best programming languages out there. After that I'd recommend Oz (in conjunction with the textbook Concepts, Techniques, and Models of Computer Programming, which Peter Norvig mentions as being a possible modern equivalent to SICP (Structure and Interpretation of Computer Programs, a classic MIT Scheme-based text)), Scheme (with SICP), and Erlang.

Can't say I've done all this myself, it's just what I would do if I had a really strong interest in learning new programming languages.