For Python hackers: check out Psyco and Pyrex. Very interesting ways to speed up Python code when you're working heavily with arrays or numbers and can't make progress with the usual methods (like a better algorithm, for example).
Psyco is a form of just-in-time compiler for Python, but it includes rudimentary type inference — it determines at the time of code analysis what values can actually arrive. This of course helps primarily in areas where specializations are included in this compiler. Current focus areas are arrays and lists of basic types, especially numbers, and arithmetic expressions.
Pyrex is something completely different, but no less exciting. Pyrex is its own language that is very closely modeled on Python, but combined with C. This makes creating Python extensions much easier than with C or C++ alone, because you program in a way familiar to Python users, but can work with C type declarations.