We have seven people in a smallish office and typically two people discuss a problem by shouting across the room, while another person is busy making phone calls. As a developer it can be hard to concentrate on programming with all this noise around. That is until I discovered the Inception app.
From the ">official website:
Experience your life as a dream [..] This app is a dream machine that transforms the world around you into a dreamworld. It uses augmented sound to induce dreams through the headset of your iPhone and iPod Touch. It will change your perception of reality.
The idea of this app is that you listen to it while you do live your life, and in certain situations it unlocks interactive dreams, for example on a sunny day - or in a busy place. The still dream I'm listening to currently records audio through the microphone and playsback the samples through an echo with pitch-shift, resulting in an ever-changing soundtrack.
In my busy office with headphones on, this successfully creates an interactive soundtrack blocking all communications. I prefer it to a regular radio station and other solutions I've tried.
Update: Unfortunately this app no longer works with iOS 8. Recommendations are welcome!