HTML 5 has precise rules on how to handle all HTML elements, and how to recover from errors. Some of the new features in HTML 5 are functions for embedding audio, video, graphics, client-side data storage, and interactive documents. HTML 5 also contains new elements like <nav>, <header>, <footer>, and <figure>.
http://www.w3schools.com/html5/html5_reference.asp

When in need of alternate wording for your copy, give the Thesaurus a chance.
http://thesaurus.com/

Quickly and easily find out who is hosting any website.
http://www.whoishostingthis.com/

A comprehensive RSS reference detailing everything you need to know about Really Simple Syndication(RSS) or Rich Site Summary(RSS).
http://www.rss-specifications.com/

Complete references of CSS selectors and properties, provided by W3Schools.
http://www.w3schools.com/CSS/CSS_reference.asp

Millions of free manuals available online, for download.
http://the-manuals.com/

An open source tool for web developers, providing quick access to CSS properties, WCAG tips, mobile tips, and other useful information from a variety of specifications published by the W3C (World Wide Web Consortium).
http://www.w3.org/2009/cheatsheet/

Complete references of all PHP functions, provided by W3Schools.
http://www.w3schools.com/PHP/

The webmaster's reference library: The reference for web developers with tutorials on all aspects of web design and authoring, including JavaScript, DHTML, 3D animation, CSS, RSS, XML, design, Flash and more.
http://www.webreference.com/