Since last summer, I've been on a semi-constant search for "noise" generators to help improve my productivity. I think I first came across the concept of background noise being used to help improve concentration, and thus productivity, when I ran across focus@will and coffitivity.com. Apparently studies have been done to show that the background noise generated in a typical coffee shop can help you stay focused on a task that requires some mental concentration. Meanwhile, focus@will operates on the theory that certain types of music are more suitable for certain types of situations, like trying to focus on a task.
It doesn't stop with coffee shops though. There are all kinds of sound generators out there, not only to aid concentration and focus, but to help people relax, fall asleep, and probably other stuff. My favorites tend to be the sounds of thunderstorms or rain.