core-js is the more common of the two in our index: it runs on 15,915 websites against 182 for ScrollMagic, and 72 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
ScrollMagic is a jQuery plugin which essentially lets you use the scrollbar like a playback scrub control.
72 sites run both core-js and ScrollMagic. That is 40% of the sites using ScrollMagic, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.