MoodTurn offers an excellent collection of calm and soothing nature sounds that can help you both relax as well as concentrate at work.
At Mood Turn, you can listen to highly-realistic recordings of birds chirping, falling rain drops, ocean waves hitting the beach and quite a few other extremely relaxing natural sounds. Just open this site in a background browser tab and run your favorite sound theme in a loop for as long as you like.
I feel I am more productive on days when it’s raining outside and this site just helps me recreate that atmosphere anytime with a click. Thanks @Orli.