Noisy Typer is a free piece of software which plays typewriter sounds as you type. It runs in the background and works with all applications ( email, web, word etc ). Key sounds include, letter keys, spacebar, backspace, carriage return and scroll up and down.
Download it here ( currently OS X only ):
NOTE: to quit the app type “qaz123” ( that is a lowercase Q not G ).
You can also quit it by running Activity Monitor ( in Applications/Utilities/ ) and quitting the Noisy Typer process.