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Free Onine IDE's

Ideone
Ideone is an online compiler and debugging tool which allows you to compile source code and execute it online in more than 60 programming languages.

ShiftEdit
ShiftEdit is an online IDE for developing PHP, Ruby, HTML, CSS and JavaScript using (S)FTP, Dropbox or Google Drive.

JSFiddle
JSFiddle is an online editor for snippets built from HTML, CSS and JavaScript. It supports a number of different JavaScript frameworks.

CSSDesk
CSSDesk is an online css/html editor that allows you to enter html code in one frame css in another and see the results in a third panel. It also allows you to download your finished code as an html file.

Maqetta
Maqetta is an open source project that provides WYSIWYG visual authoring of HTML5 user interfaces. The Maqetta application itself is authored in HTML, and therefore runs in the browser without requiring additional plugins or downloads. You can run it remotely or download it and run it locally. Allows creation of interfaces for mobile devices. (You need to register with a valid email.)