ThreadX

Express Logic, Inc.
Makes ThreadX RTOS, picokernel, portable, scalable, preemption threshold, supports many processors from small microcontrollers to high-performance RISC and DSP; full IDE support from leading compiler/debugger makers. No royalty on all products.
http://www.rtos.com/

Green Hills Software, Inc.
Sells, supports ThreadX, efficient, no-royalty, full C source, robust RTOS, gives realtime embedded developers technically advanced multitasking system, picokernel gives very high-performance with very small memory needs.
http://www.ghs.com/products/rtos/threadx.html

ThreadX
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/ThreadX

Oki Selects Express Logic's ThreadX RTOS for ARM Microcontrollers
No royalty RTOS maker announces its ThreadX is selected by Oki Semiconductor as preferred for its new line of low-level 32-bit controllers using industry-leading ARM7TDMI processor, and related development systems. [Embedded Star] <small>(October 6, 2003)</small>
http://www.embeddedstar.com/press/content/2003/10/embedded10764.html