Unix's Heritage to the Open Source Community

Good design, elegant API
brk(), chdir(), chmod(), chown(), close(), creat(), exec(), exit(), fork(), fstat(), getuid(), link(), mkdir(), open(), read(), seek(), setuid(), stat(), time(), unlink(), wait(), write() by the end of 1971
Ability to modify code of OS, libs, applications
Ability to tailor your own environment
Ability to distribute patches & improvements to the Unix community
Usenix conferences in 1970s, net.v7bugs newsgroup