If you want to learn different language syntax, then Python and Ruby are the way to go. If you want to learn more about programming in general, then you should try something different than a object based imperative scripting language. Haskell is different because it is pure (doesn't have side effects), so it can be evaluated lazily. And the execellent static type system is much different than Perl. Prolog is nice, because you mostly write down what the problem is, and the compiler figures out how to solve it.
Scheme is wonderful for its minimal syntax, which enables you to really take advantage of programs that write programs (macros).
