AppShopper - Mac Apps, Mac App Store, iPad, iPhone and iPod touch app store listings, news, and price drops
AppSales - Find the best Android apps and games from Google Play that are currently discounted or free for a limited time
AppAgg - Application Aggregator
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.
TheAppStore.org - Find apps by rating, price, category, updated date and more