I'm willing to tolerate some comment spam, as long as it is moderated so I can say 'yes' or 'no' if the content seems a bit 'off'. I've got a great set of mod_security rules that have been catching 99% of the spam comments anyway, so I'd much rather have it be easy to leave me a comment without hassling with CAPTCHA stuff
For those that have no idea what CAPTCHA is, it is the 'enter in these hard to read letters into this box so I know you are not a robot' form that you typically see on a lot of websites now.
Actually, Wikipedia has a great description
here.