Beta Family - 50,000 testers to test your app
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.
Erli Bird - For developers & testers alik
APK Pure - Download apk for Android with APKPure APK downloader. NoAds, Faster apk downloads and apk file update speed. Best of all, it's free
TestingCatalog - TestingCatalog is a community of beta testers and developers exploring android apps and exchanging feedback.
APKMirror - Android Market and APK Downloads