Glimpse - Discover trends before they're trending
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.
Trends - The fastest way to see whats trending on Github
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.
Trends.vc - Discover new markets and ideas
JSter - JSter is a community-driven catalog of the most useful javascript libraries.