Usually, for such small chips, the need for an RTOS is easily bypassed by using interrupts (like RTI) or other simpler-than-RTOS solutions that emulate basic multitasking, because (by considering code size alone) an application doesn't have enough room to get overly complex, anyway. In some situations, however, even with such small chips, an RTOS is the only reasonable way to go.
You migh elicit more responses if you add some information, like:
Do you use C or assembly (some RTOS are tailored to C coders, others to assembly fans), do you need a pre-emptive or co-operative RTOS?
Are you happy with plain round-robin, or must you have priority-based?
How many tasks do you expect to run at once?
What basic features do you expect it to have, as minimum?
Regardless, with the QG8 being rather limited memory-wise (RAM in particular), only a handful of RTOS will be able to even begin to run under it (and these not fully loaded, of course).
What size do you expect your application alone to be (RAM/ROM), without the RTOS, but considering its presense which in most cases reduces main application size?
Since I have been using my own RTOS exclusively (not free to anyone
but my highness), I haven't tried any of the free ones lurking around, but you might want to start by giving FreeRTOS a try, mentioned in this
thread.
Hope this helps.