Nature Sounds
(with comments)

This is a fun little (free) program for those who like to have some ambient background sounds going on: Atmosphere Lite.

Atmosphere Lite generates natural sounds and other audio environments on your PC and lets you listen to them in realtime, or alternatively you can record your soundscapes directly to wav or mp3 to create your own custom relaxation CDs or cassettes.

It comes with 12 presets, and you can control exactly what sounds go into each. For example, you might select the "Tropical" preset, but get rid of the random ship horn and add some night insects and bit of thunder.

For more options, you can purchase Atmosphere Deluxe for $30.