Odamex
Odamex is a free, cross-platform, modern client-server Doom engine
Odamex is a free, cross-platform (Mac, Windows, Linux) modification of the Doom engine that allows players to easily join servers dedicated to playing Doom online. The goal of Odamex is to add enhancements to Doom while still retaining that "classic Doom feel" we all know and love about the original game.