Codenvy!

Brackets is an open-source editor for web design and development built on top of web technologies such as HTML, CSS and JavaScript. The project was created and is maintained by Adobe, and is released under an MIT License.

A free, open-source Integrated Development Environment for software developers. You get all the tools you need to create professional desktop, enterprise, web, and mobile applications with the Java language, C/C++, and even dynamic languages such as PHP, JavaScript, Groovy, and Ruby. NetBeans IDE is easy to install and use straight out of the box and runs on many platforms including Windows, Linux, Mac OS X and Solaris.The download section conveniently offers prepackaged bundles/installers for each supported programming language containing only required plugins. »

IntelliJ IDEA is a free / commercial Java IDE by JetBrains. Its design is centered on programmer productivity. A number of its features are intended to accelerate development and allow programmers to concentrate on functionality while IntelliJ IDEA handles routine coding tasks.

Komodo Edit is a fast, smart, free and open-source code editor. Switching your trusty code editor is hard, but give Komodo Edit (or its big brother Komodo IDE) a try: it'll be worth your while.

Cloud9 is a code editor and development environment built with browser technology yet with the power of a desktop IDE. It's available both as an online service and as a locally hosted platform.

jsFiddle lets one easily test snippets of JavaScript/CSS/HTML code on-the-fly, adding optional libraries like jQuery.

ideone.com is a... pastebin. But a pastebin like no other on the Internet. More accurate expression would be "online mini IDE and debugging tool".

Codeanywhere is a code editor in a browser with a powerful ftp client integrated, and all popular web formats are supported (HTML, PHP, JavaScript, CSS, and XML).

Codiad is an open source, web-based, cloud IDE and code editor with minimal footprint and requirements

PythonAnywhere is a Python development and hosting environment that displays in your web browser and runs on cloud servers. The servers are already set up with everything you need: hundreds of useful Python packages and web frameworks are supported out of the box.

Koding is a developer community and cloud development environment where developers come together and code in the browser – with a real development server to run their code. Developers can work, collaborate, write and run apps without jumping through hoops and spending unnecessary money. The Cloud Development Environment

ICEcoder is a browser code editor which provides a modern approach to building websites. By allowing you to code directly within the web browser, online or offline, it means you only need one program (your browser) to develop sites, plus can test on actual web servers. After development, you can also maintain the website easily, all of which make for speedy and smart development.