RegExr is one of the best online HTML/JS based tool to learn, build and test regular expressions.
Dabblet is online tool for HTML and CSS developers for testing their code snippets.
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.
5) CSS Desk
CSSdesk is another popular online tool among CSS developers.
6) SQL Fiddle
A tool for easy online testing and sharing of database problems and their solutions. If you do not know SQL or basic database concepts, this site is not going to be very useful to you.
7) Chop App
10) HTML Edit
This is the perfect HMTL editor, paste your code in above text area and it will appear in the frame below.
12) Python Fiddle
This is the online tool for python developers.
13) Dirty Markup
DirtyMarkup combines the power of HTML Tidy, CSS Tidy, JS Beautify, and the Ace editor to effortlessly clean up your messy code. Just paste your code into the editor and press “clean”.
14) Code Pad
codepad is an online compiler/interpreter, and a simple collaboration tool.
Paste your code below, and codepad will run it and give you a short URL you can use to share it in chat or email.
15) Code Playground
16) Land of Code