Noi
Noi is a freeware noise generator and nature soundscape player for Mac OS X. Noi resides in the menu bar to instantly block out your surroundings and help you relax. It can generate random white, pink, and brown noise. Free nature sounds can be installed within the app directly from the Internet Archive's Sounds of Nature collection.

Download Noi 1.0.3 for Mac OS X 10.5.8 or later:
Noi-1.0.3.dmg