It depends on what you want to do.  Do you just want directly marketable skills?  Then learn PHP, Perl, Java, JavaScript, C++, Ruby or Python.<p>On the other hand, if you want to increase your personal skills then learn something radically different.  A variety of Lisp such as Scheme, Common Lisp or Clojure would be one option.  Another option would be OCaml or Haskell.  These are truly mind-expanding and will help you in the future.