I installed an earlier version of Typewriter Keyboard on my PowerBook G4, and while it worked as described, it hogged an inordinate amount of CPU time, so I rarely used it.

After reading the posting by chillywilly5280, I tried Keyclick on my MacBook Pro, and can confirm that when idle it uses 0.0 per cent CPU, in use typically 0.1 per cent, and even if you peck away rapidly at your keyboard, CPU use peaks at 0.3 per cent.

There's a 21-day trial period, after which you have to shell out $5.00.