MUDs are text based Multi-User-Dimensions/Dungeons played via a "Telnet" program or browser plug-in.

I'm currently helping to create a Final Fantasy series-based MUD with a turn-based combat engine, rather than the streaming combat normally found in MUDs, to make it more like the older FF games. It's not anywhere near allowing players in, but it's a fun project. It's also not including FFXI's Vana'diel either... just the older games at this point, although we may have FFXI jobs and abilities make it in.

(It's LPC based, rather than Diku or Circle, or Silly. I must admit I haven't kept up with the other MUDos's out there to know if "silly" would be worth pursuing for the project)