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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.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
Ruby Toolbox - Explore and compare open source Ruby libraries
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.
Similar Site Search - SimilarSiteSearch helps you find similar, related, or alternative websites.