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.
FindSimilarSites - FindSimilarSites is comparatively new in the list of those websites that are offering the similar, alternatives and related websites for a given website over the internet.
SimilarSites - Easily find more sites like those you like.
Product Hunt - A website that lets users share and discover new products
Similar Site Search - SimilarSiteSearch helps you find similar, related, or alternative websites.
moreofit.com - Moreofit is the first (and best) website similarity search engine.