Developer: Devin Cook

Devin Cook, also credited as Devin D. Cook, is a homebrew programmer whose first Atari 2600 game was K.O. Cruiser. He announced it on the AtariAge forums in February 2008, and AtariAge released it as a boxed 32K cartridge in 2010. It is a first-person boxing game: your boxer appears as a see-through outline, and you fight six cartoon opponents on the way to the Gold Belt.
The site has the NTSC version and the PAL60 version. Pick the NTSC page if you are unsure, and step into the ring in your browser below.