AppShopper - Mac Apps, Mac App Store, iPad, iPhone and iPod touch app store listings, news, and price drops
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
AppAgg - Application Aggregator
Cydia - An alternative app store for jailbroken devices.
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.
Fnd - Browse Apple App Store and iTunes on the Web