Free Software Directory - The Free Software Directory is a project of the Free Software Foundation (FSF) and United Nations...
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.
Softpedia - A library of over 1,000,000 free and free-to-try applications for Windows, Mac, Linux and Smartphones, Games and Drivers plus tech-focused news and reviews
Ruby Toolbox - Explore and compare open source Ruby libraries
Opensource Builders - Find open-source alternatives to commercial apps
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.