The newest version of Doomsday is fantastic! You can grab it at http://dengine.net. Just click on the icon at the bottom for your operating system.
I had a problem setting up a multiplayer server, but I finally figured it out. Here’s how to do it:
1) Make a DOOM folder and put both the Doomsday Engine and Doomsday Shell in that directory
2) Copy your DOOM.WAD and DOOM2.WAD into that directory
3) Run Doomsday Engine
4) For each game you need to locate the WAD
5) Now you can select a game and click PLAY
6) Quit the game – you’re just testing and making sure it works
7) Run Doomsday Shell
8) Open the Preferences dialog
9) Check the box “Use a custom IWAD folder” and select the DOOM folder you made
10) Go to Server > New Local Server… and name it, select the game, and click Start Server
11) Run Doomsday Engine to play on this server
12) Select NEW GAME > MULTIPLAYER > JOIN GAME > Custom Server
13) You should see your game in the list. Select it and Connect to Server
You can start up multiple servers in the Doomsday Shell by just starting more New Local Servers.