Android Essentials: What's a must have?

Hey guys,

So I just bought my first Android phone, a HTC One. Having used iOS and Windows Phone I decided to scratch my "Android Itch" and made the big purchase yesterday.

I have installed Nova launcher, and expect I will keep it. I have also installed some typical apps ( Facebook, Instagram ect.) but, so far, that's it. Having come from Windows Phone, the potential to use my phone for more than just calls and texts seems like a luxury.

So my question is: Is there anything in particular you would recommend? A new keyboard, launcher or application? A tweak or a hack? Anything to help me get the most from my phone.

In case it makes a difference, I am in Europe (Ireland) and have no problem paying for anything that seems it is worth the money.

Thanks for your help :)