AppSales - Find the best Android apps and games from Google Play that are currently discounted or free for a limited time
Amazon Appstore - The Amazon Appstore is a place to purchase and download apps for your Android device.
Fnd - Browse Apple App Store and iTunes on the Web
GetJar - Discover and play paid apps and games for free.
TheAppStore.org - Find apps by rating, price, category, updated date and more
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.