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.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
FreeSitesLike - FreeSitesLike is a user contributed website dedicated to offering its visitors with plenty of options to a specific website.
Growing Page - Browse web projects and evaluate them by multiple criteria.
SimilarSites - Easily find more sites like those you like.
PitchWall - Browse new startups, and submit your mobile app/games startup's beta page here.