A number of companies are making a push to move traditional desktop applications into the cloud. You can now write documents, spreadsheets and presentations all from the web browser with ease. Programmers have not been left out of this revolution, with several sites now offering development environments (IDEs) in a web browser.
WonderFl allows you to write, compile, run and share ActionScript applications online. It has a decent Library of projects to search through, and you can create a fork from an existing project to build on it.
Developers looking to integrate ActionScript with web based APIs can find quite a few examples in the WonderFl database like a Google AJAX search, Flickr with Yahoo Pipes, Twitter clients and more. Each of the examples can be run live directly from within the WonderFl interface.
Bespin is a Mozilla Labs experiment on how to build an extensible Web code editor using HTML 5 technology. You can sign up and use Bespin from the main site, or download the Bespin code and host it yourself.
ECCO is an online programming development environment with file manager, console interface for compiling and execution of code and an editor with syntax highlighting, tabs and support for many languages which is accessed using a browser.