I need something in the background to drown out other noises in the house or office.
For the background stuff, if I just want to focus, I load up Simply Noise, either pink sound on ossolate, or the thunderstorm. If I do the thunderstorm right I hack myself into thinking there really is a storm going.
http://simplynoise.com/+1
I have the TMSoft WhiteNoise app on my iPhone, and thunderstorms + noise-canceling headphones are a great combination when traveling, studying in public areas, etc.