Ruby Toolbox - Explore and compare open source Ruby libraries
UserBit - A full-stack qualitative research platform for UX & product teams.
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.
Rage Grade by FullStory - Benchmark your customer frustration
UXsniff - Track user behavior on your website and improve conversions with UXsniff: the user experience watchdog that looks out for you.
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.