Nice question (albeit ambiguous). I will try to be as expansive as i can be. I will give you my top 5 apps that you can download from the Play Store (which now has over a million apps), i will avoid the really common ones like Whatsapp, Viber, etc for obvious reasons.
# 5: Battery+
After using the lite version for a while i shifted to the paid version. A really great app that shows you your battery statistics, and allows you to carry-out diagnostics if you see that the drain is on the heavy side. You can also customise it in many great ways, and the graph included will help you save and plan on your battery really well,
At first i didn't give it much thought, but I find myself using this app a lot nowadays. Just customise it with the currencies you normally want to convert to or from and all this info is on your fingertips, no-one can cheat you of your money as it has the latest conversion rates. Its a great, unassuming app that really delivers,
With people operating so many accounts (from your Google/gmail account to TechZim account) you find either you have a lot of passwords to remember or you have 1 password that you use for all your accounts. Both are dangerous for a number of reasons. It's easy to lose a lot of these passwords and may find yourself locked out of your own account or if a person gets their hands on the one password imagine what harm they can bring to you. I use dashlane for these and many other reasons. I can be as creative as i can be with creating passwords without any risk of forgetting any, and it can sync across all of my devices really well. The fact that it stores the passwords on the device and not on the cloud is a great plus. You can carry-out a password audit to see how strong your passwords are and fix any issues easily. See my future posts i will try to give a lot on this app,
Ever felt annoyed by your phone ringing in an important meeting and when you put it on silent then forget to put it back up when you're done then end up missing more calls? Shush is a great app for that. Just put it on and using the scroll rocker provided set how long you want it on silent, that's it. After the time set it will come up to your normal ring and you don't have to remember to do it yourself. Has served me well so far and i use it everyday,
# 1: Google Keep
The best note-taking app by far. Simple to use, sync across devices and pull up. With the latest update you can even set reminders to any notes. I find myself on this app more than five times a day everyday, setting things i should remember to research on later, things to buy, things to see. You can even take a picture of something to check out later as a note. Thats super cool.
I hope my top 5 will give you an idea of some the really useful apps you can get from the store and use on your phone. There are others like Pocket, Goggles, Intelligent Ringer, etc that are also good. Best advice; explore as much as you can on the store.