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.
EduDo - EduDo is a video discussion platform
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
Courseroot - Biggest online course database on the web
APKMirror - Android Market and APK Downloads
TeenUp - A learning platform designed for teens by teens