[t-3]

There isn't really anything comparable - there's too much cruft and too many platforms, devices, and peripherals to support, so the simple close-to-the-hardware OS isn't all that practical anymore. Maybe one of the realtime operating systems for microcontrollers are simple enough? Parallax has some products that are fun to play around with as well as being technically interesting and quite capable (they're discontinuing the P2 eval board in favor of their modules, so you can get a decent deal right now, too).