Bukkit

It's a little box that teleports its passengers randomly around the map! It couldn't get any simpler. Unfortunately it's not bigger on the inside, but it also can't lag you out.

Mechanics:

You (the admin) will spawn a fixed number of Tardises in any world you want them, for example 3. They will teleport randomly every couple of minutes and if someone finds one, they can get in and take a ride. 

Players can also obtain Tardis Keys from endermen which can be used to summon a Tardis. All teleports stay in the same world, so it will only work in those worlds where you set them free.

Once inside, click on the sign to start the teleportation. The Tardis will choose coordinates and take you there, being careful not to break anything at its destination. It will transport any number of people, animals and items you can cram into it!

The teleport destinations are chosen in a circle with definable radius; at 10000 blocks there is a nice chance you will run into someone else's home and make friends!

It's also great for taking new players into untouched wilderness.

Commands:

/tardis: Lists existing Tardises

/tardis [x]: Teleports the user to the specified Tardis

/tardis summon [x]: Teleports the specified Tardis to the user

/tardis spawn: Creates a new tardis

/tardis del [x]: Deletes and despawns a Tardis

/tardis key: Spawns a Tardis key into the user's inventory

Permissions:

tardis.tardis: Allows the user to access the /tardis command. (Required for all commands)

tardis.tp: Allows you to list and tp to Tardises.
tardis.spawn: Allows you to spawn a new Tardis.
tardis.delete: Allows you to delete a Tardis.
tardis.key: Allows you to spawn a Tardis Key item.
tardis.summon: Allows you to summon a Tardis.

Gallery:

A Tardis arrives in an unexplored Jungle

Right-clicking the sign causes it to choose new coordinates