Microkernel OS used in realtime and embedded environments. Sun Microsystems open sourced it, with 4 licenses: Sun Public License Lite, GPL, Mozilla, FreeBSD. Description, download. [ExperimentalStuff.com]
Makes ChorusOS: highly scalable, reliable, POSIX compliant, embedded, realtime operating system; used by top telecom suppliers, in: public switches, PBXs, access networks, cross-connect switches, voice-mail systems, cellular base stations, cell-phones, web phones.