The Ruby Standard Library

http://www.ruby-doc.org/core-2.1.1/doc/standard_library_rdoc.html

Ruby Standard Library API

These are the API documents for the standard library classes and modules in version 2.1
The 2.1.1 standard library (this is the current official release)
Additional libraries included with the standard Ruby distribution, such as CGI, OpenURI, and REXML

http://ruby-doc.com/docs/ProgrammingRuby/
Ruby comes ``out of the box'' with a large and useful library of modules and classes. This chapter contains a sampling of the more useful of these.

Interestingly, and unlike some of the code in later chapters, all of these libraries are written in Ruby. You'll find the source in the lib subdirectory of the standard Ruby distribution.

Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide" 
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.