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.

GitHub Visualizer - Enter user/repo and see the project visually

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.

Codeology - Open-source algorithm that visualizes GitHub projects

GitNews Web - Trending repositories from GitHub, HackerNews & Reddit

SyntaxDB - Easily look up programming syntax for multiple languages