SimilarSites - Easily find more sites like those you like.
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
Fnd - Browse Apple App Store and iTunes on the Web
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.
Cydia - An alternative app store for jailbroken devices.