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.
GitNews Web - Trending repositories from GitHub, HackerNews & Reddit
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.
JSter - JSter is a community-driven catalog of the most useful javascript libraries.
Product Hunt - A website that lets users share and discover new products
Good First Issue - Make your first open-source contribution