What language should I learn?

Hello /prague/.

I know how to program with c, modula-2, php and java to some extent. Recommend future languages.

Python is a great choice: it allows you to quickly prototype your algorithms, and if you learn how to work with iterators and lazy evaluation it fucking rocks.
Pro: Rapid software development, battery included;
Con: Big interpreter lock prevents you from doing real threading.

Haskell is simply awesome: clean, well developed, the king of functional languages! Not clumsy as Ocaml, awesome syntax, awesome data typing...
Pro: Language from brightest minds to brightest minds;
Con: Hard learning curve on certain aspects, code must be well thought, you cannot just sketch stuff...

C++ Is simply a pain in the ass, but if you really know C as you say, by learning C++ you'll get a huge improvement of your comprension on how programming works, expecially on the operators overloading stuff. When you've really learned C++ you can throw it away because it's the best language if you want to make a mess of your software.
Pro: Extremely flexible. You can manipulate intimate parts of your program.
Con: If you manipulate intimate parts of your program without knowing what you do, you can scramble intuitive semantics, and finding errors will be a path full of sorrow and woe.

Lua Seems a toy language, and probably IS a toy language, but enchains the power of a easily embeddable turing-complete configuration language. That's why it's used in some remarkable cases (like World of Warcraft or Nmap). You are more likely to write C extensions of Lua than actually programming in it...
Pro: Simple and extremely lightweight.
Con: Error prone. By example, everything not defined just evaluates as null