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.
PitchWall - Browse new startups, and submit your mobile app/games startup's beta page here.
Ruby Toolbox - Explore and compare open source Ruby libraries
Growing Page - Browse web projects and evaluate them by multiple criteria.
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.
Product Hunt - A website that lets users share and discover new products