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
SimilarSites - Easily find more sites like those you like.
Slant.co - Slant is a collaboratively edited resource that helps you quickly make decisions.
Similar Site Search - SimilarSiteSearch helps you find similar, related, or alternative websites.
Softpedia - A library of over 1,000,000 free and free-to-try applications for Windows, Mac, Linux and Smartphones, Games and Drivers plus tech-focused news and reviews