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.
Wonder for Slack - Remember the things your team forgets
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.
Qikipedia - Instant Wikipedia summary cards on any web page
JSter - JSter is a community-driven catalog of the most useful javascript libraries.
Anycomplete - The magic of Google Autocomplete while you're typing