It really depends on the strengths of the particular language.

For example, I wrote a compact disc bookkeeping system (heavy on parsing text files) when I learned Perl; I learned Ruby and Rails by writing an auction site; I learned Fortran by writing a nonlinear least-squares fitter; I learned Erlang by writing a chat server...

The list goes on, but you get the picture. Pick a language based on its being the right tool for a particular job, then do that job. :)