Commercial CAPTCHA breakers for sale
lafdc.com
lafdc.com
Adding little bits of salt-and-pepper noise, light-colored lines, and other cutesy antics do nothing.
The simple fact is that machines are better at recognizing individual letters and characters than humans are.
millions of example images solved.
Notice how images without proper 'rectangle-size letters' like the Google ones are hard to crack because it's hard for a simple line searching algorithm to find a rectangle box to split the letter. Random lines through the letters, like in Yahoo's also mess with it.
The key is to make letters human readable, but not completely obvious to a captcha reader where one letter ends and one letter begins.