Here are some websites that provide background sounds for sleeping, working, or other sound-masking needs. They allow some customization of the sound; and they do not involve long-term streaming of MP3s (as this is an issue in many workplaces or for users with slow networks); they generate the sound within the browser, saving bandwidth.
I'll update this list as I find other sites.
- SimplyNoise and SimplyRain - be sure to check out all the controls besides the volume
level; for example, in the Rain version, the Thunder button has multiple modes
- Sound Sleeping -
lets you choose from a variety of sounds and music loops
- Buddha Machine Wall - make you own mix of loops
- GoMix.It - see the Help button for more information
These next sites don't allow for much customization, but are still good: