Web/iOS/Android: We've talked about services that help you relax or help you be more productive with the sounds of falling rain, coffee shop buzz, or multiple soundscapes that you can choose, but if singing birds helps you focus (like it does for me), Birdsong.fm delivers in droves.
At first blush, Birdsong.fm is simple: an animated background and some singing birds in the background, maybe accompanied by a few other nature sounds. You could just leave it there and go back to work—that would be enough if you work indoors, face a grey cubicle wall, or it's the dead of winter and you could use a dash of spring.
Click the menu button in the top right to see more options. From there you can grab the free iPhone and iPad app so you don't have to be tethered to your computer (additional sounds and landscapes are available in the app as in-app purchases), or you can click to buy a high-quality mp3 of the audio so you can listen to it anytime, anywhere, on any device. Finally, if you fork over your email address, you get access to the "Nest," which gives you access to additional soundscapes, including a summer scene, a rainforest, and a few others, along with first access to new soundscapes as they're added. Hit the link below to give the service a spin.
Update: Android users can get in on the fun too—the Birdsong.fm app just launched at Google Play.