PHP Documentation – http://www.php.net/manual/en/
Python Documentation – http://www.python.org/doc/
Ruby Language Documentation – http://www.ruby-lang.org/en/documentation/
Ruby on Rails Documentation – http://rubyonrails.org/documentation
C# Programming Guide – http://msdn.microsoft.com/en-us/library/67ef8sbd(v=vs.80).aspx
Node.js Documentation – http://nodejs.org/docs/latest/
CSS Reference – https://developer.mozilla.org/en/CSS/CSS_Reference
HTML Reference – https://developer.mozilla.org/en/HTML
HTML5 Reference – https://developer.mozilla.org/en/HTML/HTML5
Schema Reference – http://schema.org/
Javascript Guide – https://developer.mozilla.org/en/JavaScript/Guide
jQuery – http://docs.jquery.com/Main_Page
IMPROVED jQuery Documentation- http://jqapi.com/
LESS (CSS Preprocessor) – http://lesscss.org/
SASS (CSS Preprocessor) Documentation – http://sass-lang.com/docs.html
YII Framework Documentation – http://www.yiiframework.com/doc/
CodeIgniter Framework User Guide – http://codeigniter.com/user_guide/
Zend Framework Documentation – http://framework.zend.com/docs/overview
CakePHP Framework 2.0 Documentation – http://book.cakephp.org/2.0/en/
Symfony Framework Documentation – http://www.symfony-project.org/doc/
WordPress Documentation – http://codex.wordpress.org/
MySQL Documentation Manuals – http://dev.mysql.com/doc/index.html
MariaDB – http://kb.askmonty.org/en/mariadb
MongoDB Manual – http://www.mongodb.org/display/DOCS/Manual
CouchDB – http://couchdb.apache.org/docs/index.html
Oracle Documentation – http://www.oracle.com/technetwork/indexes/documentation/index.html
SQLite Documentation – http://www.sqlite.org/docs.html
Redis Documentation – http://redis.io/documentation
Memcached Wiki – http://code.google.com/p/memcached/wiki/NewStart
APC (Alternative PHP Cache) – http://php.net/manual/en/book.apc.php
AutoHotKey Documentation – http://www.autohotkey.com/docs/
SublimeText Documentation – http://sublimetext.info/docs/en/
TextMate Documentation – http://manual.macromates.com/en/
Adobe Photoshop Documentation – http://www.adobe.com/support/documentation/en/photoshop/
Facebook API/Graph Documentation – http://developers.facebook.com/docs/
Twitter API Documentation – https://dev.twitter.com/docs
Google+ API Docs – https://developers.google.com/+/api/
GitHub API Documentation – http://develop.github.com/
Foursquare API Docs – https://developer.foursquare.com/docs/
linkedin API Docs – https://developer.linkedin.com/
DocHub | Instant Documentation Search for CSS, HTML, Javascript, jQuery, PHP, Python, and DOM – http://dochub.io
Phing Build Scripts User Guide (PHP) – http://www.phing.info/docs/guide/stable/
MediaWiki (Wikipedia) Formatting Guide – http://www.mediawiki.org/wiki/Help:Formatting
MarkDown Syntax Guide – http://daringfireball.net/projects/markdown/syntax
Smarty Templates Documentation – http://www.smarty.net/documentation
Programming Code Q&A – http://stackoverflow.com/
Programmers Q&A (Usually non-code specific questions) – http://webmasters.stackexchange.com/
Code Review Q&A – http://codereview.stackexchange.com/
Database Admin Q&A – http://dba.stackexchange.com/
Computer Q&A – http://superuser.com/
Server Q&A – http://serverfault.com/
Unix & Linux Q&A – http://unix.stackexchange.com/
Wordpress Q&A – http://wordpress.stackexchange.com/
Webmasters Q&A – http://webmasters.stackexchange.com/
Graphic Design Q&A – http://graphicdesign.stackexchange.com/