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AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
IzzyOnDroid - This is a repository of apps to be used with F-Droid.
Cydia - An alternative app store for jailbroken devices.
F-Droid - F-Droid is an interesting alternative for Android users who want to try something different and not have to use the Goole Play store all the time.
Fnd - Browse Apple App Store and iTunes on the Web
TheAppStore.org - Find apps by rating, price, category, updated date and more