Ardence Embedded OEM Development Platform
Ardence, a Citrix Company, has embedded solutions that include RTX, ETS and ReadyOn. RTX is a real-time solution that enhances the control of Windows®, providing greater flexibility and reliability. ETS is the smallest-footprint Win32 real-time operating system in the market, and ReadyOn provides instant-on functionality for Windows® devices.

Embedded OEM Development Platform

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Nucleus OS
Nucleus OS is a full-featured, royalty fee, very efficient operating system offering broad support for networking, graphics, USB and secure communications. Its modular architecture provides specific functionality in clearly delineated components, each with a well-defined API.
preview: http://www.mentor.com  

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C Kernel
The C Kernel is a real-time preemptive kernel specially written for embedded applications running on various micro controller environments. Supports x86, 8051, C166, ZiLOG, Fujitsu F2MC-16 Since The C Kernel is written in C it is easy to make new bindings (ports) to other compilers and micro controllers or processors. GNU Lesser GPL License.
preview: http://home.hetnet.nl  

Microsoft Windows Embedded - Try it free!

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Fiasco
Fiasco is a preemptible real-time kernel supporting hard priorities. It uses non-blocking synchronization for its kernel objects. This guarantees priority inheritance and makes sure that runnable high-priority processes never block waiting for lower-priority processes. Supports x86 architecture.
preview: http://os.inf.tu-dresden.de  

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MaRTE OS - Minimal Real-Time Operating System for Embedded Applications
MaRTE OS is a real-time kernel for embedded applications that follows the Minimal Real-Time POSIX.13 subset. Most of its code is written in Ada with some C and assembler parts. It allows software cross-development of Ada and C applications using the GNU compilers Gnat and Gcc. Supports x86.
preview: http://marte.unican.es  

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rtmk - real-time microkernel
rtmk is free real-time microkernel developed by Johan Rydberg. It shared alot of ideas with the Mach microkernel developed by CMU in the late 80's and early 90's (project stopped in -94).
preview: http://rtmk.sourceforge.net  

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S.Ha.R.K. (Soft Hard Real-Time Kernel)
S.Ha.R.K. is a dynamic configurable kernel architecture designed for supporting hard, soft, and non real-time applications with interchangeable scheduling algorithms. The kernel is fully modular in terms of scheduling policies, aperiodic servers, and concurrency control protocols, which typically are not modular in traditional operating systems.
preview: http://shark.sssup.it