Android Apps...which are MUST HAVES?

So, I'm fiddling about with my new smartphone and trying to decide which apps I want to clutter it up with. 

 

My main ones are usually Whatsap, audible, Instagram, saynoto0870, k-9, etc (usually I go for utility types) as my main ones but just got to wondering what everyone else can't do without on their phones.

 

Any apps that you would recommend?  (Book related or non book related. )

 

I dip in and out of the games and tv viewing type apps but mostly I stick to what I know and feel sure I'm missing out on some great apps.

 

I'm wondering what's the best ereader.  Calibre?  Moon Reader?  Something else? What app do you use to catalog your books?  Or do you even do that?

 

I'm tired of sticking to my usual boring ones and with so many to choose from it's easy to get overwhelmed, so I thought some of you might have some stellar suggestions of MUST HAVE apps :)

 

Googleplay has just too many to choose from.  I'm spoiled for choice.