Chaos is a fairly primitive Java-based Sonic the Hedgehog 2 map editor. What makes it unique is that, unlike in the past where people had to modify savestates to create levels, this editor was the first to modify the levels in the ROM itself.
Incidentally, this code was a product of a bunch of work I did to reverse-engineer the level compression scheme in Sonic 2. I’ve summarized this algorithm on the Sonic 2 Compression Scheme page.
You can find the project on Github.