I too, will cast a vote for CMX when preemptive multitasking is required. At the time I purchased it, it had the best performance and lowest interrupt latency (which is what I required), but the documentation was abysmal. Hopefully, they've fixed that problem.

If your requirements are for a small footprint and you can get by with cooperative multitasking, Salvo is quite good. It's configurability is outstanding, allowing you to enable only the features you need. Salvo's documentation is superb.