TestFlight: The easiest and most used tool to distribute apps to external testers.
Google Play Beta Testing: Alpha/Beta test app distribution for Android applications.
FireBase: Provides you with a streamlined solution for distributing apps, with a single toolset for iOS and Android.
Appaloosa: A simple and secure enterprise app store.
AppCenter: can be integrated with your Android, iOS, macOS, tvOS, or Windows app.
TestFairy: a cross-platform solution that supports iOS, Android, Xamarin, React Native, Cordova applications.