Alpaca is a small multitasking operating system for Z-80 computers, specifically for Pac-Man/Pengo arcade hardware. It is an expansion upon my PTUI project, which was originally just an experiment to see how much of a real GUI can be put into the tight constraints of Pac-Man arcade machine hardware. The limitations are a total of 1kb of RAM (for storage and stack), 16k of ROM, sprite/tile based video hardware (1k color, 1k character ram), joystick, and two buttons.
Mind you, it doesn't do anything useful yet. (There apparently isn't enough space in a Pac-Man machine to fit an OS and applications, at least without optimising it further.)
And here is an extensive page on coding for the Pac-Man/Pengo hardware, including technical info, links to tools and sample ROM sets (no games, but there's an audio sequencer or two there; I'm sure there are some avant-garde applications for one of those).
It's interesting to look at the memory maps and other documents. In some ways the arcade machines were conceptually similar to 8-bit computers; in other ways, however, they're bizarre (for example, the Pac-Man machine's video RAM is all over the place).
Actually, there's plenty of space. There's 16k of ROM space in a basic Pac machine, and the OS will only take 1k when complete. The reason it doesn't do anything useful yet is that I haven't finished the OS let alone any applications. hehehe. :D