I would like to recommend an app that has been so important to me that it has replaced part of my brain.
Evernote is my go-to note-taking application. It saves my shopping lists, my guitar tab, my plans for an upcoming trip to Hawaii, and even a list of common questions I get from my students. I have Evernote installed on my computer, my ipad, and my cellphone. I'm always astonished that many of my students haven't used Evernote yet, so that's why I wanted to recommend it here.