Teleportation Pad

Teleportation Pads (also called Transporters) are transportation devices in World of Warcraft.

Booty Bay
To use this transporter, Horde players must first accept a quest from, a vendor in Orgrimmar (ask a guard for an Engineering trainer), at around character level 30. The quest requires you to talk to the goblin named in front of the transporter in Booty Bay who gives you another quest to receive a  which must be in your inventory if you want to use the transporter. Then, you can just step onto the pad at will to be transported to Gnomeregan.

It is not necessary to first complete the Rig Wars quest.

Alliance players cannot get and do these quests.

Gnomeregan
This is the destination of the Booty Bay transporter. Players with the Goblin Transponder can use this to transport back to Booty Bay. See Booty Bay above.

Gadgetzan
Only Gnomish Engineers with engineering at level 260 can use this transporter. And it's actually only a destination - players must first create the, which can be used anywhere to transport the player to the Gadgetzan Transporter. Usually.

This is clearly the most useful of the two Engineering transporters, since it takes forever to get to Gadgetzan, which has a neutral AH.

Everlook
Only Goblin Engineers can use this transporter. And it's actually only a destination - players must first create the, which can be used anywhere to transport the player to the Everlook Transporter. Usually.

This is good for higher level players who quest and grind in Winterspring. It's a good transport to get to the northern part of the continent.

Toshley's Station
Only Gnomish Engineers with engineering at level 350 can use this transporter. And it's actually only a destination - players must first create the, which can be used anywhere to transport the player to the Toshley's Station Transporter.

Area 52
Only Goblin Engineers with engineering at level 350 can use this transporter. And it's actually only a destination - players must first create the, which can be used anywhere to transport the player to the Area 52 Transporter.

Garm Teleporter to K3
Transporter Power Cell lets player charge the Teleportation Pad on Garm's Rise in the Opening the Backdoor in the Storm Peaks that teleports the player to a teleporter in K3.

Other known teleporters

 * Goblin Teleporters in the Deadmines.
 * Gallywix's Personal Teleporter - between Atal'Dazar and Gallywix Pleasure Palace in Azshara in the quest Chasing Gallywix.

Trivia

 * Ever watch Star Trek? Then you know what this is, at least in principle.