Reference Links
http://robotframework.googlecode.com/hg/doc/userguide/RobotFrameworkUserGuide.html?r=2.8.4
http://robotframework.googlecode.com/hg/doc/libraries/
http://robotframework.googlecode.com/hg/doc/libraries/BuiltIn.html?r=2.8.4
http://robotframework.googlecode.com/hg/doc/libraries/Collections.html
http://robotframework.googlecode.com/hg/doc/libraries/String.html
https://twiki.cern.ch/twiki/bin/view/EMI/RobotFrameworkAdvancedGuide
My favorite url is this one, it is easy to search keywords.
http://rfdocs.org/browse/robot-2.8.5/