Online tools

http://en.linoit.com/ - Sticky and Photo Sharing for you

https://www.guerrillamail.com/ - Disposable Temporary E-Mail Address
https://join.me/ - free screen sharing and online meetings
http://gyazo.com/en - screen grabbing tool
http://www.online-convert.com/ - Convert media files online from one format into another
http://caniuse.com - Compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers
http://www.ustream.tv - live streaming platform
http://new.livestream.com/ -  live streaming platform
http://www.online-stopwatch.com/ - Online Stopwatch tool
http://www.hidemyass.com/ - web proxy
http://goo.gl/ - Google URL Shortener
http://webwhiteboard.com - online whiteboard
http://qrcode.kaywa.com/ - QR code generator
http://2epub.com - converts various e-book formats.
http://2zip.org - allows to extract archives online and convert many 'exotic' formats to ZIP.
http://puzzleware.net/codehtmler/default.aspx - is a simple program that translates plain text code into a colorized HTML version of the code
http://www.diffchecker.com/ - online text diff

Online development tools

http://writecodeonline.com/php/ - php playground

http://sandbox.onlinephpfunctions.com/ - php playground
http://jsbeautifier.org/ - javascript beautifier 

http://regex101.com/ - regexp playground
http://regexpal.com/ - a JavaScript regular expression tester
http://jsoneditoronline.org/ - json online editor
http://rubular.com/ - regexp playground
http://gskinner.com/RegExr/ - regexp playground
http://www.debuggex.com/ - regexp playground
http://www.regexper.com/ - regexp playground
http://regexponline.com/ - regexp playground
http://jsfiddle.net/ - online editor for snippets build from HTML, CSS and JavaScript
http://sqlfiddle.com/ - a tool for easy online testing and sharing of database problems and their solutions.
http://refiddle.com/ - regexp playground
https://koding.com/ - software development in the browser
http://codepen.io/ - is all about front end code inspiration and education through sharing
http://dabblet.com -  is an interactive playground for quickly testing snippets of CSS and HTML code.
http://cssdesk.com/ - css playground
http://jsdo.it/ - is an online editor where you can try running your codes as you write.
https://tinker.io/ -  is a quick and easy tool for writing and sharing code
http://tinkerbin.com/ - lets you play around with HTML, JavaScript and CSS without creating files or uploading to servers
https://thimble.webmaker.org/en-US/editor - makes it ridiculously simple to create your own web pages. Write and edit HTML and CSS right in your browser. Instantly preview your work. Then host and share your finished pages with a single click. Easy
http://liveweave.com - is a HTML5, CSS3 & JavaScript playground
http://code.google.com/apis/ajax/playground/ - Code Playground
https://compilr.com - online platform that allows developers to write and learn code all from the comfort of their web browser
https://c9.io/ - is an online platform for development; the code is open-sourced on GitHub, free to adapt and use for everyone, anywhere, anytime.
https://api.divshot.com - work easier with twitter Bootstrap
http://www.bestwebfonts.com/ - web fonts
http://editor.method.ac/ - SVG editor
http://cedvel.com/ - application for designing grid systems
http://cssdeck.com/ - is a place where you can quickly create some experiments (or testcases) that involves HTML, CSS, JS code.

Do you know some useful online tools?