This app runs in the background on your app and makes typewriter noises while you type. It picks up keyboard input from any app your working in, meaning that you can be sure and annoy the heck out of everyone around you no matter what you are doing!
Seriously, though, we think this is a very fun app, and it’s free. The sounds are accurate, and though there is a slight delay between what you type and what you hear, it’s almost imperceptible. Here, you can hear it for yourself in this demonstration video:
Noisy Typer Demo Video by Theo Watson of FAT Lab
We do have one pedantic nit to pick (as noted by a commenter on Vimeo), and that is the bell that sounds when you hit return. On mechanical typewriters, the bell sounded at a certain point on the page to remind you to hit return, rather than being a product of the return itself.
Who cares, though?! It makes your Mac sound like a typewriter! Huzzah!