Programming Languages

C or C++: For when performance is the #1 concern

Java: Best all round programming language for non-toy programs.  Verbosity prevents 'shortcuts' that are useful for toy programs.

Mathematica: When you need to crunch some numbers.

Anything else: Only acceptable for toy or web applications.
Haskell: Never acceptable.

NO EXCEPTIONS

>>20
Common Lisp (Object System) or CLOS has true generic methods which allow multiple inheritance(multi-methods), and at the same time classes can inherit from multiple superclasses. The actual data structures (classes and instances of them) are separate objects from generic functions, thus this works very well in practice and makes many ``design patterns'' become unneeded/obsolete (things like the Visitor pattern which is simply needed to work around the limitation of single dispatch object systems is a common example). Such an implementation is much more generic than most other so-called ``OO'' languages as it does not tie methods with classes directly. Aside from multiple inheritance, one has many ways to do method combinations and mix-in's.