I was just (a couple of days ago, but it’s still loaded in my browser) an article from 2007 titled “Stopping spambots with hashes and honeypots” which outlines various ways of preventing spam without bothering users (well, most users, some few do see the honeypots).

I personally don’t mind the occasional simple sum, or easy task (“what’s the square root of -1?” is one I saw recently, others I’ve seen include entering the domain of the website). In fact, what annoys me more is having to enter an email address when I intend to merely post a throw away comment, or where I don’t intend to ever have a conversation with the author. In such cases I normally put a fake email address. I do not know what the point of requiring an email address is.