The Cortex system is used in all of the RPGs for licensed properties produced by Margaret Weis Productions.
The bones of the Cortex system were first seen (but not named) in the Sovereign Stone RPG.
The system was then apparently reworked for the Serenity RPG, where it was formally named. In fact the name derives from the Cortex of the Firefly 'verse.
The system was evolved again for use in the Battlestar Galactica RPG, which may be where the Cortex Plus designation originated.
In-print games using the system include the Leverage RPG, the Smallville RPG, the Supernatural RPG, and the Marvel Heroic RPG.
Leverage and Smallville use modified versions of the system called Cortex Plus Action and Cortex Plus Drama, respectively. Marvel Heroic uses a modified version called Cortex Plus. The cortex-plus tag should be used for these systems.