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.

PitchWall - Browse new startups, and submit your mobile app/games startup's beta page here.

Ruby Toolbox - Explore and compare open source Ruby libraries

Product Hunt - A website that lets users share and discover new products

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.

SimilarPages.com - SimilarPages has found and classified millions of interesting sites hidden due to low popularity with the goal of helping you discover and search a larger portion of the web