Thanks for this list go to Stu_san and RickB who compiled the links:
A list of RTOS links for AVR processors:
FreeRTOS: A Free RTOS for microcontrollers
AvrX: Real-Time Kernel for AVR processors
YAVRTOS: Yet Another Atmel AVR Real-Time Operating System
AVRAsmOS: A tiny OS for small AVRs
pc/OS RTOS Kernel (for larger AVR processors >= mega128)
uSmartX: Non-Preemptive Priority-based Multitask RTOS
csRTOS: one of the longest 'freaks threads ever
scmRTOS: a C++ OS with Mit license
XMK (may no longer be supported)
BeRTOS (with lots of peripheral support libraries)
ChibiOS from Giovanni de Sirio.
(I'll keep editing entries into this list as they are suggested below)