Andrew Munsell has a pretty funny tale of trying to enter a captcha: those squiggly word-things on signup pages. They have become more and more difficult to solve, because as Munsell notes, computers are getting better and better at being more human with regard to solving these puzzles.
Captchas might be one of the more user-hostile things on the web, but we need them to stop SPAM and AHH, we need them!
The thing is (at the risk of sounding like a broken record) if you just charge every user for a sign up, then you never need to worry about captchas. Because even if a SPAM bot signs up, well, you get paid, and that’s not a sustainable model for spammers.