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
API List - A collective list of APIs. Build something.
RocketAPK - See related links to what you are looking for.
Abstract APIs - Simple, powerful APIs for everyday dev tasks
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.
Phantombuster - A marketplace of simple to use no-code APIs