Ruby Toolbox - Explore and compare open source Ruby libraries

Hunter TechLookup - Create lists of websites by technology for free

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.

Techmap.io - Techmap is a workplace search engine to find companies using technologies (e.g., Rails, PHP, Java, etc.) that are used internally. Finally, a "Google for Workplaces" that helps people find a place to work with technologies they love!

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.

Setalist - The easiest way to make awesome lists for the internet