What is your essential Android app?

Everyone is now accustomed to downloading large numbers of apps onto their Android devices.  All apps, however, are not born equal.  There are some apps which have become essential parts of the Android experience.

Which app has become an essential on your Android phone?

Personally, after using it for a few years, I have found that Spotify has become my essential app, that I use on a daily basis.  While the subscription based model might not be for everyone my experience of the Spotify mobile app is that it makes listening to music on the go a far simpler and more streamlined experience.