Ask HN: Ready-to-implement specifications?

Do you have good examples of clearly laid out, easy to follow specs for would-be implementers to pick up as a fun project?

I know of the Z-Machine standards[0] and CHIP-8[1].

[0]http://inform-fiction.org/zmachine/standards/z1point1/index.html [1]http://devernay.free.fr/hacks/chip8/C8TECH10.HTM