http://www.freertos.org for the freeRTOS

http://www.micrium.com for uC/OS-II -- but it is not free for commercial work.

I think the rest of the available RTOSs will at least require registration. Most of them are not free.

One I'm really beginning to like is uTasker. http://www.utasker.com It is not free for commercial work but it is inexpensive. The documentation is good -- much better me than the documentation of the freeRTOS.

to get uCOSII you have to either get it from the CD that comes with the uCOSII book MicroC OS II: The Real Time Kernel

or register with Micrium