Linux kernel
The monolithic kernel system Linus Torvalds began in 1991. World's most-installed software — Android, every web server, Steam Deck, most of IoT.
30M+ lines of code, ~85% are drivers. Developed via mailing lists (Linux Kernel Mailing List, LKML) + git (which Torvalds created in 2005 for the kernel). Release cadence: new major version every ~9–10 weeks. LTS versions supported for 6 years. Major maintainers: Torvalds (BDFL), Greg Kroah-Hartman (stable trees). Rust support added 2022 (more pragmatic landed 2024+ — Asahi Linux drivers in Rust).