AppSales - Find the best Android apps and games from Google Play that are currently discounted or free for a limited time
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
Fnd - Browse Apple App Store and iTunes on the Web
Cydia - An alternative app store for jailbroken devices.
TheAppStore.org - Find apps by rating, price, category, updated date and more
Knicket App Search - The future Wikipedia for Apps