The Cortex Plus System is a toolkit RPG system that evolved from Margaret Weis Productions, Ltd's Cortex System. It has been used for three published games and one published preview to date, and the design principles are in the Cortex Plus Hackers Guide, a book of advice in how to create new games using Cortex Plus, and list of new games produced via kickstarter. According to the Hackers' Guide there are three basic 'flavours' of Cortex Plus; Action, Drama, and Heroic.
Of the three currently published games under this system (with a fourth announced), Leverage: The Roleplaying Game was nominated for the 2011 Origins Award for best Role Playing Game, and Marvel Heroic Roleplaying won the 2013 award and the award for best support as well as the 2012 ENnie Award for Best Rules and runner up for Best Game.
Note that despite the similarity in name, Cortex Plus is not the Cortex System, also produced by Margaret Weis Productions.
Source: Wikipedia, "Cortex Plus", available under the CC-BY-SA License.
From the Smallville RPG:
The rules as a whole are known as the Cortex Plus System, and they're a variation on a set of rules used in other games published by Margaret Weis Productions, Ltd.. Cortex uses dice of various sizes, from four-sides to twelve-sides, and a primary focus is using the rules to help players tell dramatic and interesting stories. It's called Cortex Plus because... there are even more twists and tweaks than before.
When MWP switched lead designers (from Jamie Chambers to Cam Banks), the Cortex Classic system morphed into the Cortex Plus system. The system logo changed (adding the Plus). Much of the terminology stayed the same (Plot Points, Complications, Traits, and the various die types), but the mechanics changed drastically from the very traditional Cortex to the very narrative Cortex Plus.