When I need to concentrate and get some hardcore, head-down work done, I employ the use of what I call "Distraction Killers". A collection of sound generators to help get me in the zone mentally.
Ambiance: Available on Windows, OSX, iOS, and Android. This app gives you access to a complete library of free downloadable sounds that you can listen to individually or mix together to make your own distraction killer. Be careful with this one, I've gotten distracted by the sheer amount of sounds and mixes before. Really awesome app, I wish the Android version would get a visual overhaul (it really needs it), but the functionality is still there.
MyNoise.net: A huge collection of free noise generators (and some paid ones if you would like to contribute). From Jungle Life to Number Stations to Sounds from the USS Enterprise, MyNoise.net has just about everything you would want in noise generators (and they add more all the time). These sounds are very well produced and the online player comes with an "animate" feature, that subtly changes the sounds around you, so they don't become too repetitive. I highly recommend this site if you need to sit down and get some work done.
RainyCafe.com: A very simple site with two buttons: One turns on Rain sounds, the other turns on Cafe sounds. I go here quite a bit because it has the best cafe ambiance I've found (the rain isn't half-bad either).
SOUNDROWN: A simple site with a great twist: You get five sounds to turn on or off and mix the volumes as you see fit. Choose from Coffee Shop, Rain, Waves, Fire, and Birds, mix them together to make a pretty unique ambiance. I don't use this as much as the others on this list.
Rainy Mood: This list simply wouldn't be complete without Rainy Mood. The very best rain/thunderstorm sounds anywhere. I use this every night when I sleep and at work most days to drown out the activity around me. It's well-looped, expertly produced, and is a beautiful (yet simple) website as well. They also put out apps for all the major platforms that I rely on all the time. As an added bonus, their fans have suggested music to go along with the great rain ambiance, and they're listed right at the bottom of the page. I've listed some of my favorites below.
Fireplace: Very simple, just a fireplace with sound.
Piano cover of Where Is My Mind by the Pixies: Slow enough to not be distracting, but not so suble that it blends into the background.
The Fragrance of Dark Coffee : Quite possibly the classiest thing I will ever put on this blog. Slow, jazzy, saxaphone. Turn on Rainy Mood, set this song to loop, and pour yourself the most expensive scotch you have on hand. This is by far, my favorite combination.