Banks and Financial institutions. Every bank and financial institution that I have worked for in in last 10 years has a projects exclusively in either Java or C# or Cobol( yes Cobol is very much alive and kicking on IBM mainframe ) and some C/C++ . On the other hand, these places do not use Ruby or Python.
the banks using C# are most probably windows users, right? (Mono is not for enterprise.....)
I attended a confrence by a company named Micro Focus, and believe me, they were talking about renewing COBOL and teaching it to the undergraduates, making the mainframe relevant to the next generation.