Amazon Appstore - The Amazon Appstore is a place to purchase and download apps for your Android device.

AppShopper - Mac Apps, Mac App Store, iPad, iPhone and iPod touch app store listings, news, and price drops

GetJar - Discover and play paid apps and games for free.

AppAgg - Application Aggregator

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.

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.