High Performance Network IO with Python + Libevent
This is a library to wrap low-level asynchronous APIs like kqueue.
It's faster than Twisted, however it doesn't provide the whole deferred infrastructure. It's still based on callbacks.
It can be used underneath Django to make it three times faster.
It is incompatible with the Python GIL. That is, don't try to make use of it with Python threads.
There is little documentation.
They can do Comet.
If you are considering using Libevent, please read my article Concurrency and Python. Also, have a look at Eventlet. Asynchronous APIs are a good start, but there's so much more you can do on top of them than simple callbacks!