Ruby Toolbox - Explore and compare open source Ruby libraries
Product Comparisons - Compare any 2 software products
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.
TestBox - Test drive your software options before you buy
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.
Hackster - Hackster is a community dedicated to learning hardware.