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PyPy had its 1.0 release recently.

Now, This looks *really* interesting:

PyPy Stackless

PyPy can expose to its user language features similar to the ones present in Stackless Python: no recursion depth limit, and the ability to write code in a massively concurrent style. It actually exposes three different paradigms to choose from:

• Tasklets and Channels
• Greenlets
• Plain Coroutines