Another two popular app that can't be found in Google Play Store.

Humble Bundle
Another way to get paid games on the cheap, Humble Bundle offers regular bundles of games at a price that you set yourself.

AyaTube YouTube Downloader
A brand new Material Design YouTube replacement app allows you to open videos in any external player, play audio in the background, and download audio or video for local playback. Best of all, it doesn't require Google Play Services, which is a big help if you're trying to use Android without Google.