AppSales - Find the best Android apps and games from Google Play that are currently discounted or free for a limited time
Sensor Tower - Sensor Tower is a platform for app store optimization and app industry intelligence.
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
App Annie - App Annie is a marketing analytics tool available for apps of all kinds. With App Annie, you can track sales, traffic, and a variety of other factors pertinent to monitoring an app's trajectory.
TheAppStore.org - Find apps by rating, price, category, updated date and more
appfigures - Cross-platform app store analytics for all of your mobile apps.