Bandcamp - Discover amazing music and directly support the artists who make it.
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.
SoundCloud - Enjoy music & follow favourite artists
Ruby Toolbox - Explore and compare open source Ruby libraries
Pandora - Pandora Radio is a free (ad-supported) internet radio founded in 2000.
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.