This is the first of a (hopefully regular!) series of links to PythonAnywhere articles around the net.
Over at Develop in the Cloud, Cameron Laird of Phaseit, Inc., rounds up a set of online IDEs with an honourable mention for us, particularly because we "dogfood" -- that is, we use PythonAnywhere to develop PythonAnywhere.
PythonAnywhere is a Python development and hosting environment that displays in your web browser and runs on our servers. They're already set up with everything you need. It's easy to use, fast, and powerful. There's even a useful free plan.