Kolibri-A

Ostin project   (Compare Project)

Mostly written in Assembly

License: BSD-3-Clause, gpl

Kolibri OS   (Compare Project)

Mostly written in C

License: gpl

pmbsd   (Compare Project)

License: bsd

The MirOS Project   (Compare Project)

Mostly written in C

License: apache, artistic_gpl, Beerware, bsd_2clause_ish, BSD-3-Clause, BSD-3-Clause, bsd, gpl, MirOS, mit, XFree86_Project_Licence_version_1_1, zlib_libpng

coreboot   (Compare Project)

License: GPL2

SmartOS   (Compare Project)

Mostly written in JavaScript

License: Unknown

ReactOS   (Compare Project)

Mostly written in C

License: GPL2

illumos   (Compare Project)

Mostly written in C

License: BSD-3-Clause, common_development_and_distribution

AROS   (Compare Project)

Mostly written in C

License: AROS_Public_License_version_1_1

Nexenta   (Compare Project)

Mostly written in shell script

License: common_development_and_distribution, gpl