APK Studio - APK Studio is an open-source Integrated Development Environment that allows you to recompile and decompile Android applications with its unified interface.
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.
JADX - JADX is a decompilation tool that can produce Java Source code from Dex and Apk files, being capable of providing human-readable java classes, it reverses AndroidManifest.
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
Apktool - Apktool is an all-in-one tool that can extract all the resources inside an APK.
APKMirror - Android Market and APK Downloads