I've notised that Stack Overflow only presents me with a captcha occasionally. Does anyone know if it has a good algorithm for sensing if I might be a robot or, instead if it's just random.
Reducing reliance on captchas is obviously a good thing.
Stackoverflow.com uses reCAPTCHA. And, I too have noticed that when you try to edit the same question many times in a gap of a few seconds, it displays the captcha verification page.
Most of the CAPTCHA generation algorithms are strong enough. This article describes the factors influencing a CAPTCHA's strength.