Working completely online is not only a getting popular trend but also something that happens slowly in the background almost without us realizing it.
Many of us have online storage accounts to host our documents/codes, lots of web apps exist to help us accomplish our daily tasks (image editors, cross-browser testing tools, etc.).
Orion, by Eclipse, is an open source and online IDE application (that is alternative to others) which can be a good alternative for anyone willing to create code online.
It has 2 versions, hosted or self-hosted, and the self-hosted version requires Java to run.
The application supports working with multiple projects, creating files/folders, FTP and there is Firebug + Git integration.
Orion is extensible with its plugin system and it is well-documented.
While creating/editing CSS files, viewing the output instantly saves much time.
SoFresh!, a bookmarklet, extends it a little further, works without inserting a JS file to our web pages and allows us to choose which CSS file to refresh.
In case you don't have such a live-updating development environment, you should definitely give this tool a try.
Compatibility: All Major Browsers
When displaying code in a web-page, highlighting its syntax accordingly helps so much in easily reading it.
There are various syntax highlighters around (see 11 Syntax Highlighters To Beautify Code Presentation) and Lea Verou has released a lovely new one, named Prism, that is built with modern web standards in mind.
Prism is lightweight (1.5KB minified & gzipped), can be integrated so easily (just insert a CSS and JS file) and works fast. It is already used to beautify lots of code in Dabblet (an interactive CSS playground) so its pretty stable.
New languages can be added and the functionality can be improved with the plug-in architecture and the look/feel can be completely styled via CSS.
Requirements: No Requirements
Compatibility: All Modern Browsers
If you are always developing with a specific language, using a development tool that is built specifically for that language can ease the process.
NINJA-IDE is an open source and free development tool that focuses on Python developers at first but has support for code highlighting for various languages.
It has a handy code locator which enables us to access to any file, function or class inside the projects and a debugger that can highlight static and PEP8 errors in the document.
The application allows managing Python Project automatically, saving descriptive information about them and letting the user to perform file management related task in the IDE itself.
NINJA-IDE is highly extensible with its plugin support and there are already multiple plugins like a snippet manager or Git integration.
P.S. It works in all major operating systems.
Requirements: Python 2.7+, PyQt 4.7+
Compatibility: All Major OS
The editor focuses on simplicity and, rather than too many panels, the quick-edit UI is the main thing.
A "live development" feature syncs a browser with the editor and displays any changes instantly.
It is integrated with the Chrome's Developer Tools and JSLint for debugging.
Brackets, although being web-based, works in a native shell (Win & Mac versions available) to reach the filesystem.
Requirements: Windows or Mac