Linley's Dungeon Crawl aka Stone Soup. The most worked and polished old school roguelike there has ever been. Just in terms of game design, it is fascinating watching this project evolve. I have never seen a project make more fearless choices, and they almost always turn out to be winners.

http://crawl.akrasiac.org:8080/

Edit: OOPS! Not really multiplayer. There are some social features on the site that hosts it allowing you to watch other people's games and heckle them and stuff.