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.
GitSpo - Measure & engage your open-source community
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
Devo - Devo delivers real-time operational & business value from analytics on streaming and historical data to operations.
SimilarSites - Easily find more sites like those you like.
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.