I think asking really simple questions that only a human could understand seems to get the job done most of the time.
Perhaps something like: "What is the opposite of bad?" or "How many planet Earths are there?"
I've used things like this for a handful of projects and have never had any problems: https://github.com/kiskolabs/humanizer
Basically, whoever answers most of these questions correctly, earns $15,000. People submit MILLIONS of answers.
This list is then incorporated into the Xrumer itself.
I can tell you that these things are easily broken: a) Answering questions and building global list of answers, as in this case. b) Reading image captcha - spammers send it to Pakistani manual solvers for dirt cheap. c) More complex puzzle captchas can also be broken in software if a lot of websites implement them.
What works the best is using non standard html form field names. Also, try to not use text labels for the fields ( no "password", "captcha" etc. ) - because the software will try to match the best field by text surrounding it. It is better to use image for labels.
This solution will stop spambots because they simply match form field names. Unless someone specifically targets your website, in which case there's not much you can do.
From the most common of captchas, Mollom seemed to be the biggest pain in the ass for spammers. Mainly because it banned suspicious IPs ( you could solve the picture correctly and it wouldn't authorize it. )
Except browser autofill breaks and anyone who needs a screen reader will go elsewhere if the screen reader can't parse the images.
"Either one or probably infinity, depending on whether the universe is infinitely sized and whether unobservable parts of it are considered to 'exist'."
What is the most widely used project you have successfully used it on?
They have created interesting methods to combat Captcha images. They even have outsourced OCR services, where people in other countries are paid to solve ocr 1 image at a time. [0]
That's the hard thing about captchas, bots don't have to solve them perfectly to completely break them.