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As noted in “A Quarter Century of Unix”, Usenet played an important role for Unix users to exchange bug fixes and third-party source code, and just to communicate with each other. Here are links to some relevant newsgroups.
comp.sources.bugs: Bug fixes to systems and applications
comp.sources.games: Source code to games, some of which run on Unix
comp.sources.misc: Miscellaneous source code, some of which runs on Unix
comp.sources.reviewed: Peer reviewed source code, some of which runs on Unix
comp.sources.sun: Third-party source code for Sun systems
comp.sources.unix: Third-party source code for Unix systems
comp.sources.x: Source code related to the X Windowing System
net.v7bugs: Bug fixes for 7th Edition Unix