This is a list of six android apps, some of which are popular and some of which are not very popular, but all of these apps are much less popular than I think they should be. All of the listed apps are free:
AirDroid: File Transfer/Manage
"AirDroid: File Transfer/Manage" does a lot of things. It mirrors many apps onto your computer and it allows easy file transfers. The app also includes extra security including remote access to the front and rear camera and a "find my phone" feature.
Google Opinion Rewards
"Google Opinion Rewards" is an app used to make money by taking quick surveys posted by Google. The surveys are very simple with questions like "Which of these stores have you been to recently?". Each survey you do gives you Google Play Credit. In my experience the survey rewards span between 10 and 40 cents and the number of questions span between 2 and 10 questions. I have made about five dollars in the past two months. You get a notification whenever there is a new survey available.
CamScanner - Phone PDF Creator
Lower Brightness Screen Filter
"Lower Brightness Screen Filter" is exactly what it sounds like. It is an app that lowers the brightness on your phone. The awesome part is it'll lower the brightness of your phone to lower than the lowest brightness. This is for when your in a really dark room and your phone brightness is at its lowest but still too bright. It is very simple and easy to use. Also it only takes up 3.1 megabytes.