The App Store is now supporting unlisted apps. This software can only be accessed via a direct link, according to Macrumors, picked up on a tip from developer Steve Troughton-Smith.
App Store Supports Unlisted Apps
The details are outlined on the Apple developer website. To release an unlisted app, developers must submit a request. This can be done for both new and existing software, but it must be ready for final distribution, not in beta. If granted, the app will not appear in App Store listings, including categories, recommendations, or search results.