ARM Mbed OS - The Arm® Mbed™ IoT Device Platform provides the operating system, cloud services, tools and developer ecosystem to make the creation and deployment of commercial, standards-based IoT solutions possible at scale.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
LynxOS - LynxOS is a Unix-like real-time operating system designed for controlling and monitoring laboratory instruments.
ClearOS - A very good alternative for Windows Server, for free
MikroTik RouterOS - The main product of MikroTik is a Linux-based operating system known as MikroTik RouterOS.
Univention Corporate Server - Univention Corporate Server UCS with Active Directory functions and App Center is the cost-efficient and popular successor to Microsoft's server solutions.