You can customize a never-ending cat purr on Purrli. If you're away from your cats, or severely allergic to cats but wish to hear their cute sounds, you can use Purrli to customize a purr that will play out loud to your heart's content. This experience was way more comforting than I expected.
I'm definitely going to take a nap while listening later.
You can adjust the distance, engery, steadiness, sleepiness, and other purr qualities by sliding the various bars. There's even a feature that throws in a little meow every now and then (and my cats keep looking around everytime it plays).