Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Noisli - Noisli is a fantastic background noise and color generator for working and relaxing. Online soothing ambient sounds like White noise, Rain and Coffee Shop.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Noizio - Feeling that the noise of the big city is too much for you to bear?
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
zenmix.io - Ambient background sound generator