I'm currently studying a range of different types of
languages:
- LISP (bigloo)
- Ada (gnat - available on Linux and, I think, Win32)
IMHO, a C/C++ equivalent both in terms of what it can do
and in how much you have to know to use it well. But,
IMNSHO, much better than C/C++ because of its
international standardization and strong type checking,
among other things.
- Ocaml
A derivative of ML (which someone else mentioned).
- Mathematica