What's your most valued app/service?

Apple-app-store-apps_medium

Everyone seems to have that one piece of software that they love. You know the one. Every setting has been tweaked, every menu closely examined, every feature memorized. Yeah, that one.

Do you have a ton of playlists in Spotify? Have you backed up your brain with Evernote? Can't go anywhere without checking in on Foursquare? I want to hear about it.

Sound off in the comments with your killer app/service!