Arduino Uno
The current wave of lliputing owes a lot, if not all, to Arduino boards. Of course, a lot has changed since Arduino's first days and a big chunk of Arduino stuff is, today, even too much powerful and too much cloudy (i.e. connected to Arduino cloud) to be really interesting.
But you can still buy simpler boards - like Arduino Uno, the (more or less) original one, and start interesting projects on them. In the collapse/frugal compuing sense, they're perfectly viable systems. And if you don't like the Arduino C-like language, there are other ways. It's just an example, but you can have a little Arduino board with Lisp on it. Well, MicroLisp.