Ruby Standard Library
The Ruby Standard Library is a vast collection of classes and modules that you can require in your code for additional features.
Below is an overview of libraries and extensions followed by a brief description.
Libraries
- Abbrev
 - 
Calculates a set of unique abbreviations for a given set of strings
 - Base64
 - 
Support for encoding and decoding binary data using a Base64 representation
 - Benchmark
 - 
Provides methods to measure and report the time used to execute code
 - CGI
 - 
Support for the Common Gateway Interface protocol
 - DEBUGGER__
 - 
Debugging functionality for Ruby
 - Delegator
 - 
Provides three abilities to delegate method calls to an object
 - DRb
 - 
Distributed object system for Ruby
 - English.rb
 - 
Require ‘English.rb’ to reference global variables with less cryptic names
 - ERB
 - 
An easy to use but powerful templating system for Ruby
 - Find
 - 
This module supports top-down traversal of a set of file paths
 - GetoptLong
 - 
Parse command line options similar to the GNU C getopt_long()
 - MakeMakefile
 - 
Module used to generate a Makefile for C extensions
 - Monitor
 - 
Provides an object or module to use safely by more than one thread
 - Net::FTP
 - 
Support for the File Transfer Protocol
 - Net::HTTP
 - 
HTTP client api for Ruby
 - Net::IMAP
 - 
Ruby client api for Internet Message Access Protocol
 - Net::POP3
 - 
Ruby client library for POP3
 - Net::SMTP
 - 
Simple Mail Transfer Protocol client library for Ruby
 - Observable
 - 
Provides a mechanism for publish/subscribe pattern in Ruby
 - OpenURI
 - 
An easy-to-use wrapper for Net::HTTP, Net::HTTPS and Net::FTP
 - Open3
 - 
Provides access to stdin, stdout and stderr when running other programs
 - OptionParser
 - 
Ruby-oriented class for command-line option analysis
 - PP
 - 
Provides a PrettyPrinter for Ruby objects
 - PrettyPrinter
 - 
Implements a pretty printing algorithm for readable structure
 - profile.rb
 - 
Runs the Ruby Profiler__
 - Profiler__
 - 
Provides a way to profile your Ruby application
 - PStore
 - 
Implements a file based persistence mechanism based on a Hash
 - Racc
 - 
A LALR(1) parser generator written in Ruby.
 - RbConfig
 - 
Information of your configure and build of Ruby
 - resolv-replace.rb
 - 
Replace Socket DNS with Resolv
 - Resolv
 - 
Thread-aware DNS resolver library in Ruby
 - Rinda
 - 
The Linda distributed computing paradigm in Ruby
 - Gem
 - 
Package management framework for Ruby
 - SecureRandom
 - 
Interface for secure random number generator
 - Set
 - 
Provides a class to deal with collections of unordered, unique values
 - Shellwords
 - 
Manipulates strings with word parsing rules of UNIX Bourne shell
 - Singleton
 - 
Implementation of the Singleton pattern for Ruby
 - Tempfile
 - 
A utility class for managing temporary files
 - Time
 - 
Extends the Time class with methods for parsing and conversion
 - Timeout
 - 
Auto-terminate potentially long-running operations in Ruby
 - tmpdir.rb
 - 
Extends the Dir class to manage the OS temporary file path
 - TSort
 - 
Topological sorting using Tarjan’s algorithm
 - un.rb
 - 
Utilities to replace common UNIX commands
 - URI
 - 
A Ruby module providing support for Uniform Resource Identifiers
 - WeakRef
 - 
Allows a referenced object to be garbage-collected
 - YAML
 - 
Ruby client library for the Psych YAML implementation
 
Extensions
- Coverage
 - 
Provides coverage measurement for Ruby
 - Digest
 - 
Provides a framework for message digest libraries
 - IO
 - 
Extensions for Ruby IO class, including #wait and ::console
 - NKF
 - 
Ruby extension for Network Kanji Filter
 - objspace
 - 
Extends ObjectSpace module to add methods for internal statistics
 - Pathname
 - 
Representation of the name of a file or directory on the filesystem
 - PTY
 - 
Creates and manages pseudo terminals
 - Readline
 - 
Provides an interface for GNU Readline and Edit Line (libedit)
 - Ripper
 - 
Provides an interface for parsing Ruby programs into S-expressions
 - Socket
 - 
Access underlying OS socket implementations
 - Syslog
 - 
Ruby interface for the POSIX system logging facility
 - WIN32OLE
 - 
Provides an interface for OLE Automation in Ruby
 
Default gems
Libraries
- Bundler
 - 
Manage your Ruby application’s gem dependencies
 - CMath
 - 
Provides Trigonometric and Transcendental functions for complex numbers
 - CSV
 - 
Provides an interface to read and write CSV files and data
 - E2MM
 - 
Module for defining custom exceptions with specific messages
 - FileUtils
 - 
Several file utility methods for copying, moving, removing, etc
 - Forwardable
 - 
Provides delegation of specified methods to a designated object
 - IPAddr
 - 
Provides methods to manipulate IPv4 and IPv6 IP addresses
 - IRB
 - 
Interactive Ruby command-line tool for REPL (Read Eval Print Loop)
 - Logger
 - 
Provides a simple logging utility for outputting messages
 - Matrix
 - 
Represents a mathematical matrix.
 - Mutex_m
 - 
Mixin to extend objects to be handled like a Mutex
 - OpenStruct
 - 
Class to build custom data structures, similar to a Hash
 - Prime
 - 
Prime numbers and factorization library
 - RDoc
 - 
Produces HTML and command-line documentation for Ruby
 - REXML
 - 
An XML toolkit for Ruby
 - RSS
 - 
Family of libraries that support various formats of XML “feeds”
 - Scanf
 - 
A Ruby implementation of the C function scanf(3)
 - Shell
 - 
An idiomatic Ruby interface for common UNIX shell commands
 - Synchronizer
 - 
A module that provides a two-phase lock with a counter
 - ThreadsWait
 - 
Watches for termination of multiple threads
 - Tracer
 - 
Outputs a source level execution trace of a Ruby program
 - WEBrick
 - 
An HTTP server toolkit for Ruby
 
Extensions
- BigDecimal
 - 
Provides arbitrary-precision floating point decimal arithmetic
 - Date
 - 
A subclass of Object includes Comparable module for handling dates
 - DateTime
 - 
Subclass of Date to handling dates, hours, minutes, seconds, offsets
 - DBM
 - 
Provides a wrapper for the UNIX-style Database Manager Library
 - Etc
 - 
Provides access to information typically stored in UNIX /etc directory
 - Fcntl
 - 
Loads constants defined in the OS fcntl.h C header file
 - Fiddle
 - 
A libffi wrapper for Ruby
 - GDBM
 - 
Ruby extension for the GNU dbm (gdbm) library
 - IO::console
 - 
Console interface
 - JSON
 - 
Implements Javascript Object Notation for Ruby
 - OpenSSL
 - 
Provides SSL, TLS and general purpose cryptography for Ruby
 - Psych
 - 
A YAML parser and emitter for Ruby
 - SDBM
 - 
Provides a simple file-based key-value store with String keys and values
 - StringIO
 - 
Pseudo I/O on String objects
 - StringScanner
 - 
Provides lexical scanning operations on a String
 - Zlib
 - 
Ruby interface for the zlib compression/decompression library
 
Bundled gems
Libraries
- DidYouMean
 - 
“Did you mean?” experience in Ruby
 - MiniTest
 - 
A test suite with TDD, BDD, mocking and benchmarking
 - Net::Telnet
 - 
Telnet client library for Ruby
 - PowerAssert
 - 
Power Assert for Ruby.
 - Rake
 - 
Ruby build program with capabilities similar to make
 - Test::Unit
 - 
A compatibility layer for MiniTest
 - XMLRPC
 - 
Remote Procedure Call over HTTP support for Ruby