AppCrawlr - Discovering new applications
Cydia - An alternative app store for jailbroken devices.
AppShopper - Mac Apps, Mac App Store, iPad, iPhone and iPod touch app store listings, news, and price drops
Fnd - Browse Apple App Store and iTunes on the Web
BlackBerry App World - Get apps for your BlackBerry with App World.
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.