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.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
Product Hunt - A website that lets users share and discover new products
TechStorey - Discover a new Tech Business at the click of a button
SimilarSites - Easily find more sites like those you like.