A multiplayer test of the sequel to Wolfenstein3d, "Return to Castle Wolfenstein" has been released. It uses the Quake3 engine and looks really good. This is a test though not a demo, then full version will be mostly based on singleplayer but it will have some multiplayer features too. I recommended you try this out if your computer can handle it and you have enough space. If you can run Quake3 and it runs smooth then you shouldn't have much of a problem running this.
There are some mirrors at: http://www.castlewolfenstein.com
just click the link on the left that says multiplayer test for the mirrors. The servers are probably loaded (earlier, fileplanet went down because so many people were accessing the page) so just keep on trying.