I heartily recommend the mod zenNonCAPTCHA. Instead of the clunky usual CAPTCHA technique of forcing a user to type a string or identify images (which is often quite difficult for older users), zenNonCAPTCHA is done with a slider test. Moving the slider until the value “Human” is shown is how you pass the CAPTCHA test. Here’s a screenshot of how the slider looks when it is first presented:
And here’s how the slider looks when you have successfully moved it to Human: