ScrollMagic - The jQuery plugin for magical scroll interactions.
AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
WOW.js - Animate elements on-scroll to catch attention. Such Magic.
Ruby Toolbox - Explore and compare open source Ruby libraries
Lax.js - Create beautiful & smooth animations when you scroll
LibHunt - LibHunt tracks mentions of software libraries on relevant social networks. Based on that data, you can find the most popular projects and their alternatives.