Software for which the source code (set of instructions written in a human-readable programming language) is freely distributed.

• Boston Consulting Group/OSDN Hacker Survey - surveyed hackers participating on software projects on SourceForge and the Linux kernel mailing list
• CollabNet - provides platforms and services based on open source tools and principles, empowering companies to create global, collaborative software development environments
• Eric Raymond's Home Page
• Free Software Foundation - dedicated to eliminating restrictions on copying, redistribution, understanding, and modification of computer programs, by promoting the development and use of free software - in particular, the GNU operating system
• Freshmeat - extremely active source of OSS code announcements and downloads
• Open Source for E-Government
• Open Source, Open Standards - argues that the success of an open source project depends on open standards, which clarity shared vision of the goals of the software and some strong definitions of basic functions and how they will work, Karen Coyle, Information Technology and Libraries, March 2002
• Open Source Initiative
• The Open-Source Revolution, Release 1.0, Tim O'Reilly (introduction by Esther Dyson), November 1998
• Open Source Software: A History - traces the evolution of three operating systems: GNU, Berkeley Software Distribution (BSD), and Linux, as well as the communities that have evolved with these systems and some of the commonly used software packages developed using the open source model, David Bretthauer, Information Technology and Libraries, March 2002
• Open Sources: Voices from the Open Source Revolution - book available online, 1999
• Open Source Systems for Libraries (oss4lib) - "the communications outpost for folks working with open source systems in libraries."
• OpenSource Timeline - 1969-Present - Web Review
• O'Reilly Open Source Center
• Possibilities for Open Source Software in Libraries - enumerates a number of possibilities for open source software (OSS) in libraries and how it can be leveraged to provide better and more effective digital library collections and services, Eric Lease Morgan, Information Technology and Libraries, March 2002
• Softpanorama: Open Source Software Educational Society
• SourceForge - services for open-source development, including CVS, mailing lists, bug tracking, message boards/forums, task management, site hosting, permanent file archival, full backups, and total web-based administration