I'm rather old school when it comes to my cell phone. For the most part I use it to make calls and send a text or two.
Since I began writing though, I ended up with a few apps I use frequently.
WhatsApp - Since "meeting" Italian model Luca Pantini, it's our go-to way to communicate/text as long as he has good cellphone reception.
Voxer - it's a pretty cool little tool, has a Walkie / Talkie feature and texting feature. I'm sure it has many others. I'm using the free version right now for a project I'm working on.
Instagram - I never had an account until I became an author. I use my cellphone to upload pictures, and my laptop for comments and everything else.
Then there was my failed attempt with the MLB app when trying to secure some Cubs tickets for a game in May. What a nightmare that was! Never fear though, we were able to get our tickets at the box office and see the game - Go Cubs!!
What about you? Any apps you can't live without?