Go to the Universal Access preference pane, and click on the keyboard tab, then turn on Slow Keys (the last preference on the pane). Move the slider called "Acceptance Delay" all the way to "short" (the far right side of the slider) so that it won't delay the keys at all. Make sure the checkbox named "Use key click sounds is enabled."
Its completely useless, but there is something satisfying about the manual typewriter sound it makes.
[Editor's note: It took me a few minutes to figure out why this wasn't working on either of the machines I tested it on ... you also need to enable "Play user interface sound effects" in the Sound control panel, otherwise you won't hear the typewriter sound ... and I agree with the author - perhaps useless, but for those of us who grew up typing on a 'real' typewriter, it's a good sound to hear!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021115062039393