What is your favourite App? [iOS & Android welcome!]

Hello,

I have been searching around and have found a large amount of people asking so I decided to ask everyone in general... What is your favourite app, it doesn't matter if it's for iOS, Android, Mac or any-other OS... Just let us know what your favourite app is, maybe give a little description and other users may be persuaded to go and download that App!

My current favourite app is Wunderlist I currently use it on my Mac's and iPhone and I find it great, it has an endless amount of uses and it syncs between all of your devices and the web... So anywhere you are you can always view your lists.

So guys, let us know; What is your favourite app?