Twisted is described as 'event-driven network programming framework written in Python and licensed under the MIT License' and is an app in the Development category. There are three alternatives to Twisted for a variety of platforms, including Linux, Mac, Windows, Self-Hosted solutions and BSD. The best alternative is Node.js, which is both free and Open Source. Other great apps like Twisted are Opa (Free, Open Source) and asyncoro (Free, Open Source).
Opa is an open source, simple and unified platform for writing web applications. All aspects are directly written in Opa: Frontend code, backend code, database queries and configuration. And everything is strongly statically typed.
asyncoro is a Python framework for developing concurrent, distributed programs with asynchronous completions and coroutines. Asynchronous completions implemented in asyncoro are socket I/O operations (non-blocking sockets), database cursors, sleep timers and locking primitives.