I lived through the transition from typewriters, to the brief era of the word processor, and finally to the era of the computer keyboard. Call me nostalgic, but I still find the sound of an old fashioned mechanical typewriter as pleasant as the sound of raindrops. There's now an OS X app to add this sound of yesteryear's to every keystroke...
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.
I know I'm probably imagining it, but I swear my typing accuracy improved once I heard the sound of a typewriter rather than the usual tic-tacking of the Apple chiclet style wireless keyboard. It could have been I'm simply just having fun hearing the sound, adding a little fun to a mundane everyday task.
Download NoisyTyper.app here ( currently OS X only ): NoisyTyper-v001.zip