9C Sound Machine
See also: Online Brown Noise Generator

Free. Plays white noise, and others, perhaps to drown out the neighbors' garbage music playing at midnight or whatever you may need it for. I'm not currently maintaining a Linux version any longer however the Windows version works perfectly in Wine. Using the rate dial may completely reverse desirability/undesirability for certain sounds to some individuals, so feel free to experiment with it. The 46Hz generator creates a pleasant rumble when used with a decent subwoofer that, in my case, completely silences all sounds from adjacent apartments with the speaker in a corner of the room, and cannot be heard at all when I'm not near a wall. Without a subwoofer it will likely have no effect and you might not even hear it.

Uses:
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Drowning out neighbors
Calming yourself, kids, students, coworkers, etc
ADD/ADHD Focus
Inducing sleepiness (all ages)
Deeper sleeping
Tinnitus suppression
Localizing conversations
Quieting fireworks/other sounds for panicking pets
Silencing misophonia trigger sounds




Downloads for Windows 7/8/10/11:

Compact 0.07MB - procedurally generated sounds only
Standard 3.09MB - has built-in samples as well
Android version soon.