I don't yet have any need to implement CAPTCHA myself, but if I did, it wouldn't be your standard distorted and scribbled on text. It would be one of these:
With Asirra, to identify yourself as a human you have to identify a series of pictures as cats (excluding the dogs). It seems like a sound approach, but on the face it looks so nonsensical that I feel compelled to use it. Picture this internet argument: “Well you plainly have no idea what you're talking about on this issue, so I won't keep wasting my valuable time trying to fight with your stupidity! As soon as I click on these cats, you'll never hear from me again!”
This one is more serious, and has a purpose too. Instead of displaying random obscured characters, it displays a real scanned image of two words from an old book. One of these words has been identified and the other has not. The user types both words, the computer verifies the user's humanity with the known word, and records what the human said the unknown word was. Through this process the un-digitized book becomes completely digitized. They're turning CAPTCHA tests, a “lesser evil” annoyance, into something that's actually good.
Cezar: We use reCAPTCHA at work. It's pretty good. The best thing is that it supports switching the images or making and audio captcha for the sight impaired.