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
Sites Like - Siteslike opened it's doors to the public on the 4th of december in 2008, since that time...
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.
SimilarSites - Easily find more sites like those you like.