Raspberry Pi, Arduino and their board-level Maker-centric ilk. It's a viable flea-market ecosystem now that everybody already has at least a computer of some sort, so the learning machine doesn't have to be that do-all hub-of-activity machine, it just has to connect to it.

Having GPIO pins to play with also helps with the learning. The Boca Raton model of computing device was a bit stifling for homebrew hardware extension because of having to accommodate the de facto standards of the IBM PC/AT model (RS232 serial, Centronics parallel, CGA/EGA/VGA display, DIN5/PS2 kybd/mouse) to get a hackable connection going.