Stackless Python is an experimental implementation of the Python language; Stackless was designed from the start to overcome the limitations of cPython's Global Interpreter Lock by using tasklets, which implement functions as microthreads.
My incomplete understanding is that Twisted, Stackless, Greenlet, Eventlet, Coroutines all make use of async network IO and userland threads …
python asynchronous erlang nonblocking python-stackless