Orb of Translocation
There are five known connected pairs of orbs in Azeroth and Outland. The first and most-often used connects Sunfury Spire in Silvermoon City to the Ruins of Lordaeron in the Undercity, which was probably implemented to allow blood elf players in Quel'Thalas to travel to the "mainlands" without having to venture through the Eastern Plaguelands. Alliance players cannot use this orb. This is probably to prevent easy raids on Silvermoon City.
Translocation is also used in single buildings to connect from the ground floor to the top. Such locations include Duskwither Spire in Eversong Woods, Falcon Watch in Hellfire Peninsula, the Sun's Reach Sanctum on the Isle of Quel'Danas, and the Kael'thas-controlled Firewing Point in Terokkar Forest.