Matching Decompilations — Atari (Game Decompilations & Reimplementations): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Reverse-engineered, fully documented 6502 source of Star Raiders
Reverse-engineered, commented source of Raiders of the Lost Ark for Atari 2600
Comprehensive disassembly of Mario Bros. for the Atari 2600