Ruby Toolbox - Explore and compare open source Ruby libraries
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.
Product Hunt - A website that lets users share and discover new products
UserBit - A full-stack qualitative research platform for UX & product teams.
Alternative.me - Welcome to alternative.me, the source of better software alternatives. Finding suitable software was never easier.
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.