Eventlet

Eventlet is a networking library written in Python. It achieves high scalability and concurrency by using non-blocking io while at the same time retaining high programmer usability by using coroutines to make the non-blocking io operations appear blocking at the source code level.

Linden Lab open-sourced Eventlet in 2007, and it's been under continuous development since. Several applications have been built on top of Eventlet, such as Spawning and Proxylet.