This is the code for a mud, written in Ruby. Persistant data is stored with activerecord, and almost everything is interacted with through a model of some sort.
Just ruby server.rb - this will spawn the main eventmachine loop.
This is a fairly standard mud setup - there are Players and NPC Mobiles who are located in Rooms. Items can be located in rooms, or held by players or mobiles. Players additionally can have Buffs and Debuffs. When a player gets input, it is compared against the names of its command groups (players get "default" for free), and the names of the commands held within that group.
A lot of multi-table inheritance is needed for having different types of mobiles. To facilitate this, the owns_one relationship automatically forwards methods for a delegate table's attributes to the owner. For example, if you have MobileA < Mobile, and it needs to have the attribute feelings (string) but no other mobiles need this, you would create another model (and table) MobileADelegate, and do owns_one :mobile_a, "MobileADelegate" in MobileA. All of the base tables which might need this functionality already implement a helper to make this easy (for instance, Item defines self.item_delegate)