#6 Inferno

Year created: 1996

Company: Bell Labs

Inferno is a distributed operating system started at Bell Labs, but is now developed and maintained by Vita Nuova Holdings as free software. It was part of the Bell’s research into operating systems, languages, on-the-fly compilers, graphics, security, networking and portability. Inferno programs are portable across a broad mix of hardware, networks, and environments.

Because it's an open-source operating system, so there are versions of it still out there but they are not used much.

#5 XTS-400

Year created: 1992

Company: BAE Systems

The XTS-400 is a multi-level secure computer operating system. It works in networked environments and supports Gigabit Ethernet and both IPv4 and IPv6.The XTS-400 is a combination of Intel x86 hardware and the Secure Trusted Operating Program (STOP) operating system. XTS-400 was developed by BAE Systems, and originally released as version 6.0 in December 2003.

STOP provides high-assurance security and was the first general-purpose operating system with a Common Criteria assurance level rating of EAL5 or above.

Valued for their security, this operating system and its successors are still used in military technology.

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