ErisMUD is a free MUD/MOO server and engine written from scratch in Python using SQLite for data storage. ErisMUD is distributed under the GPL. It is a work-in-progress but already has many features:
Creating/destroying/listing rooms, items, NPCs with a single command. Moving around in all directions. Importing text maps directly into the system. ANSI colors. Basic health/damage system. Talking using the "say" command, chatting using the "chat" command. Cloning items, dropping them on the floor, getting them, giving them to other players. Instances: a single object may be cloned multiple times and every instance is independent. Easily add or modify help files with a single command. Logging.