My pick for a must-have Android app in 2013 was AirDroid, but now I have to update that selection. It was just displaced with AirDroid 2 which adds remote phone access over 3G/4G and more features.
My pick for Android app of the year? AirDroid, which provides wireless management of your Android phone or tablet from any browser on the same Wi-Fi network. You can manage files, videos, music, ringtones and more on any computer, making for a great free companion application.