Perhaps the best known fan-made Ultima project is Exult. The Exult team took apart Ultima VII's game files, wrote an entirely new engine to read the data, and made it compatible with a range of platforms, including Windows, Linux, Unix and Mac.
They even managed to revamp the graphics, and it's by far the best way to relive one's memories of the game. (Or, if you've never played Ultima VII, experience the best title in the Ultima series).
If you're looking for something with a more modern look, then the Ultima 6 Project will pique your interest. Rather than deconstruct Ultima VI and rebuild it, Ultima 6 Project is a massive total conversion for Gas Powered Games Dungeon Siege. RPGWatch has an interview up with Jesse Strachman, the project's co-lead producer:
RPGWatch: What most still impresses you about the game?
JS: The sheer scope of what we're trying to do is just amazing. We look at the original Dungeon Siege game, and then we look at our own maps and all the dungeons, and I think our world is like 25 times the size or something absurd like that. It's just so vast. Even with what little we've done so far, you still have probably, what, 30-50 hours of playtime? Every time I sit down to furnish another house and define another NPC's schedule, I just have to stop and think, wow. I have to do this another 300 times before we're done. Now that's impressive.
While milestone builds are available to download and play (the latest being the 375MB Milestone 4), Strachman believes the whole shebang won't be complete until late 2009.
I couldn't find any information regarding multiplayer options, but Dungeon Siege has ample support, so I wouldn't be surprised if it made it into the final mod.