Khorium Power Core

Khorium Power Core is made by engineers as a part for some high-level items they craft.

Source
Recipe is trainable at any Grand Master Engineer trainer, requires engineering skill 340.

Also, Qiff in Area 52 in Netherstorm sometimes has this as a Limited Supply item for.

Engineering

 * 2x Khorium Power Core for

Gnomish engineering

 * 1x Khorium Power Core for
 * 2x Khorium Power Core for
 * 4x Khorium Power Core for

Goblin engineering

 * 1x Khorium Power Core for
 * 1x Khorium Power Core for

Also, Khorium Power Cores were used to craft Remote Mail Terminal in Burning Crusade beta. However, this recipe seems removed in release version of the game.