The Cathedral and the Bazaar, Eric Raymond's seminal statement of what we already dimly knew but didn't see the implications of: https://www.catb.org/~esr/writings/cathedral-bazaar/index.html
Cygnus Solutions: https://www.cygnus.com/
eCos home page: https://sourceware.cygnus.com/ecos/
Free Software Foundation: https://www.fsf.org/
Jet Propulsion Lab: https://www.jpl.nasa.gov/; Mars Pathfinder https://mars.jpl.nasa.gov/default.html; Voyager https://vraptor.jpl.nasa.gov/voyager/voyager.html
Linux Embedded: a listing of Linux distributions intended for real-time or embedded applications at https://linux-embedded.com/
O'Reilly & Associates: https://www.ora.com/; especially DiBona, Ockman and Stone, eds, Open Sources, Voices from the Open Source Revolution: https://www.oreilly.com/catalog/opensources/ (reviewed in the July 1999 Linux Journal, page 24)
RTLinux: https://www.rtlinux.org/~rtlinux/