Whatsapp : Almost everyone knows this, but this will keep you in touch with people and save you a lot of calling money
Truecaller : Let you know the identity of callers, even the unknown callers and spammers
Es file Explorer : You can easily manage all your file and even automatically save the files you view on instagram, whatsapp status, facebook and others.
Microsoft Office : Microsoft is a must have software on PC, the mobile version can do almost everything the computer version.
Duolingo : To translate and learn many new languages