If you're looking to get out of your comfort zone, but want to learn something that has some practical value, consider learning Haskell, Forth, or a hardware description language such as Verilog or VHDL.
Haskell for a pure functional language with modern syntax. It eliminates the sometimes-annoying parentheses of LISP syntax, gives you a lot more modern functional programming language features, but it also allows no side effects -- thus enabling lazy evaluation.
Forth for one of the only really successful low-level/embedded-systems programming languages after C and assembly languages. Which can express very high level ideas concisely, but usually with arcane trickery.
Verilog/VHDL for a taste of languages that model inherently parallel processes. See http://www.fpga4fun.com/ for some starting points. (The common use of such things is to create designs implemented in FPGAs or ASICs.)
A little closer to your comfort zone: Have you spent much time with C or C++? (I'd recommend C++ and judicious use of the STL, even if you're not going for highly-OOP design, because it's nice to have some useful, well-tested container classes on hand.)