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.
SimilarSites - Easily find more sites like those you like.
Growing Page - Browse web projects and evaluate them by multiple criteria.
Product Hunt - A website that lets users share and discover new products
PitchWall - Browse new startups, and submit your mobile app/games startup's beta page here.
Similar Site Search - SimilarSiteSearch helps you find similar, related, or alternative websites.