A linear-feedback shift register, the pseudo-random source behind the SID noise channel and countless games. Set the width and taps, step it, and watch the bits cycle. Click a bit to change the seed.
Taps are the polynomial exponents (the highest, equal to the width, is implied). A maximal set runs through every non-zero state, 2n − 1 of them, before repeating. The all-zero state is a dead end, which is why hardware seeds it with something else.