GeckOS/A65 – preemptive multitasking OS for the 6502 (6502.org)

Also see the author's other articles:

http://www.6502.org/users/andre/icapos/mp.html http://www.6502.org/users/andre/icapos/osa65.html

And this discussion on the 6502 forums for more info on multitasking on the 6502:

http://forum.6502.org/viewtopic.php?f=2&t=2281