Here is one example of the captcha I use:
It's customizable to the point where I can add noise, lines, change the font to any true type font I provide, and exclude or include certain characters, numbers, and symbols. Also, once thing I notice about it's implementation is that the images that appear are generated dynamically with php, so they are not generated and stored as image files -- meaning that once the page has loaded and the captcha displayed, no image file exists. So the embed code for the captcha image would look like this:
<img src="/image_captcha/1939250506" />
But that's not an image file so when when you try to load that in a browser, you get nothing.
On the other side of things, I also use reCAPTCHA
for another site, which seems to be extremely effective. reCAPTCHA aims at providing captcha challenges to prevent spam, but also to digitize books by having humans translate the scanned words into text. Some of the letters are completely unrecognizable as individual letters, but because the letters are in full words, the human brain can understand.