StumbleUpon, aka "discovery engine" has integrated app discovery into its Android app's repertoire. This feature suggests apps you'll like based on what apps you've already installed and recently used, what apps your friends on StumbleUpon like, and a host of other factors.
To use, just download the app by searching "StumbleUpon" in Android Market, or scan the QR code below to directly download the app. Once installed, you simply tap the button labeled "Apps." You'll then be able to stumble through, rate, review, and share apps recommend for you, just like you can do with web pages on your phone or computer. When you see an app that interests you, just tap "Install" and you'll be taken directly to the app's download page in the Android Market.
There're a number of 3rd party sites and apps, such as Chomp for iPhone or AppAware for Android, among others, which attempt to help people discover relevant and useful apps. Apple also offers its "genius" recommendations technology for apps within iTunes.