There's lots of different iterations of the Cortex system. Cortex Classic is the Firefly and Battlestar Galactica RPGs, and is basically a progenitor of Cortex Plus Action (the Leverage RPG).
The Cortex Hackers Guide is a filing off of the serial numbers from the Cortex Plus Action (Leverage), Dramatic (Smallville), and Heroic (Marvel Heroic RP) versions of the system, and ports them each into generic or non-IP settings. The C+HG had all the system information necessary to play those three settings/systems, plus it shows you how to mix and match there unique elements for your own setting hacks.
There is also now Cortex Prime (a unification of the Action/Dramatic/Heroic "package" into one modular toolset) which successfully kicked more than a year ago, but the guy who took over is dragging his feet in a major way. And there's a lot of uncertainty as to whether it's ever going to actually see the light of day, although updates do still trickle out at a glacial pace.
I wish I had time to run something new right now.