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.
Product Hunt - A website that lets users share and discover new products
Ruby Toolbox - Explore and compare open source Ruby libraries
Free Software Directory - The Free Software Directory is a project of the Free Software Foundation (FSF) and United Nations...
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.
Filehippo - Filehippo is your key to opening files that Windows claims are "in use by another user" or simply "can't be found." Avoid the hassle.