JSON Lint is a web based validator and reformatter for JSON, a lightweight data-interchange format.
Browsershots makes screenshots of your web design in different operating systems and browsers. It is a free open-source online web application providing developers a convenient way to test their website's browser compatibility in one place.
The Responsinator is designed to test responsive websites on different device resolutions.
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.