I assume you mean "learn" as in "master", not just merely pick up the syntax to bang out the next big web app(tm) quickly, as it's fairly easy to pick up Python/Ruby/PHP/Javascript/whatever if you already program in a mainstream language.

Programming languages:

Haskell - Lots of interesting language features that you don't see elsewhere

Assembler - Already know some C, but would like to be working knowledge of the inner workings of computers (especially things like cache misses etc)

R - Need to brush up on statistical analysis skills

Natural language: Already learned English... I guess I'd take Japanese or Korean next? It'd be mostly for entertainment.