First answer - Scheme. It's not too widely used, but definitely seems like a solid language to use, especially considering the robustness of DrScheme (which in fact compiles Scheme programs to native binary code).
After that - Haskell is incredibly interesting. It's a language which does lazy evaluation right, and the consequences are incredible (including such things as a one-line definition of the fibonnaci sequence).
Going more mainstream, Python is still not really widely accepted in the business circles, but it definitely should be, by now...
