If I want to just bang/try something out or show somebody an idea or concept, I go Python every time. The syntax just gets out of the way and I don't have to be as picky about all the details. If I want to be sure it works I go to Haskell. Basically, if it compiles it works, and I've brought the techniques I learned from Haskell into other typed languages to be able to reap some of the benefits there as well. If it needs to be fast, C/C++ or Fortran.