Noizio was launched a little over a year ago but I've only recently stumbled across it. It is a wonderful little app for OS X and iOS that sits in your dock and plays relaxing ambient sounds. At the moment, I'm sat here listening to gentle October rain, wind chimes, and the rumblings of a distant thunderstorm. If you prefer, you can also listen to a bustling coffee shop, a crackling campfire, the rhythmic beat of a train, or the now clichéd whale songs.
When I need to work I find both silence and noise equally distracting and I frequently work in both types of environments: from the silence of my study at home to, ironically, the noise of a train carriage full of passengers. I'm also fussy about the background music I listen to - anything too lively distracts me. I don't have the time to curate my own playlist that is long enough to get me through the working day and manually switching between albums is yet another distraction.
At the moment, Noizio seems to be working well for me. I can fire it up in the morning and it just trundles along in the background emitting soothing sounds. The best part is that it's free!