NeptunIDE!

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.

Smart PHP Code Editor

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.

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".

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

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.

ShiftEdit is a free web based IDE.Features: Syntax highlighting, Integrated (S)FTP, Tabs, Find/Replace, Revision history, bookmarks.

CodeLobster - Free portable PHP IDE with support Drupal, Smarty, WordPress, Joomla, JQuery, CodeIgniter, HTML, CSS, JavaScript, CakePHP, Facebook, Symfony

Powerful, Collaborative Online/Offline web IDE

Powerful PHP IDE and PHP editor with built-in HTML, CSS and JavaScript editor. Fast. Powerful. Time-saving!

Codio for Everyone:

Online-php.com is a free online IDE for editing files on an FTP server. The application was created for fast, free, simple and convenient read/write access to the files on FTP server. Sometimes, for some purposes, the direct FTP access is blocked and there is no conventional way to access the files on the server. This is the main reason for this WebApp.

A powerful, feature packed, fully configurable IDE specifically designed for Windows developers.