Other

Older operating systems which are still used in niche markets include OS/2 from IBM and Microsoft; Mac OS, the non-Unix precursor to Apple's Mac OS X; BeOS; XTS-300. Some, most notably AmigaOS 4 and RISC OS, continue to be developed as minority platforms for enthusiast communities and specialist applications. OpenVMS formerly from DEC, is still under active development by Hewlett-Packard. There were a number of operating systems for 8 bit computers - Apple's DOS (Disk Operating System) 3.2 & 3.3 for Apple II, ProDOS, UCSD, CP/M - available for various 8 and 16 bit environments.
Research and development of new operating systems continues. GNU Hurd is designed to be backwards compatible with Unix, but with enhanced functionality and a microkernel architecture. Singularity is a project at Microsoft Research to develop an operating system with better memory protection based on the .Net managed code model. Systems development follows the same model used by other Software development, which involves maintainers, version control "trees", forks, "patches", and specifications. From the AT&T-Berkeley lawsuit the new unencumbered systems were based on 4.4BSD which forked as FreeBSD and NetBSD efforts to replace missing code after the Unix wars. Recent forks include DragonFly BSD and Darwin from BSD Unix.