Freedoom is a free implementation of the Doom data files. It allows the Doom engines in Debian to reside in main. It's playable, but gameplay and graphics are not yet as balanced and polished as the original from Id Software.
They're releasing new versions every 6-12 months, the packaging is very simple. Unfortunately upstream uses a web board instead of a mailing list, but the main maintainer is reachable by email and responsive.