Elsewhere: Ambient Nature Sounds is a nice little free app that lives on your menu bar and plays ambient sounds in the background while you work or relax.
There are three environments; City, Forest and Beach. Each soundscape has a switch for nighttime or daytime ambience, or the app can change automatically based on your time zone.
Elsewhere's sounds seem of high quality, but I think they would best be appreciated on external speakers. You can add the sounds of rain to any of the environments for US$0.99, but I got along fine with the free sounds. I found that in general I liked the night variant, even during the day, except in the forest where some owls or other arboreal denizens were a bit distracting.
The sounds are random, so it's not just a short loop endlessly repeating. On headphones the sound was quite good. The audio is stereo, and I thought the best directional effects were on the Beach preset. I don't know if my concentration was any better as a result of using Elsewhere, but I liked having the sound on, which works best for me if it is just above the level of audibility.
Elsewhere is a 42 MB download that requires OS X 10.7 or better.