Reactor

From StarCraftWiki

The Reactor is a Terran add-on that allows a production building (Barracks, Factory and Starport) to create two units at once. Unlike add-ons from the original Starcraft, the Reactor is able to be attached to any building that produces units (except, perhaps, the Command Center).

[edit] Drawback

Only one add-on at a time is allowed on each building. That means that any unit that requires a Tech Lab will not be available to build with the Reactor attached. Therefore, you have to choose between a large amount of weaker units or fewer, more advanced units. This further emphasizes the Terran balance between quality (Protoss) and quantity (Zerg).