First, ASP.NET has a control that isn't truly a "captcha," but in fact quite the reverse - a very simple script which makes sure that the visiting program can evaluate JavaScript. This gets rid of all but the most complex scrapers, especially if the JavaScript test has a structure that changes (i.e. it isn't just var y = 2; var x=y+(random number from server); verify(x))
Google and Craigslist both use phone numbers, which mandate that a nasty bot at least have access to an SMS-capable number (or speech recognition + voice line)
My favorite captcha is clicking on something that a computer can't recognize, such as picking out a cat from a short list of animal pictures.
It's important to consider accessibility and ease of implementation, which reCAPTCHA does very well.
