Odamex is a free Doom modification that allows users to join servers for playing Doom with other players over the internet. With so many great enhancements being coded into Odamex for version 0.7, the dev team and community felt it necessary to create a holdover release for the little open source multiplayer engine that could. Odamex 0.6.3 introduces improved ZDoom 1.23b33 compatibility, further improvements to the sound engine, the ability to mute enemies and spectators, and the ability to connect to "DOS" 1.9 servers with Freedoom and Doom 3: BFG Edition's versions of Ultimate Doom and Doom 2 (and vice versa).
You can download Odamex for Windows, Linux, and Mac OSX from the home page @ http://odamex.net
An optional build that includes the experimental "32-bit renderer" slated to be finalized for version 0.7 can be found on the Odamex forums.
Oh and don't forget to join the Steam group for Odamex here: http://steamcommunity.com/groups/odamex