GitHub Visualizer - Enter user/repo and see the project visually
Ruby Toolbox - Explore and compare open source Ruby libraries
SyntaxDB - Easily look up programming syntax for multiple languages
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.
Gource - Gource is a software version control visualization tool.
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.