I don't know how relevant it is to your situation, but I'd just like to share my CAPTCHA solution.
There is no CAPTCHA on registration. Instead, users are required to play a little bit of the game that they have signed up for, before they will be allowed to send message and post on the forums.
This means that anyone wishing to bot their way onto the site will have to build a custom-tailored bot capable of playing a game up to a certain point in the tutorial. If they're willing to invest that much effort in specifically targeting my site, then a CAPTCHA wouldn't do much good anyway because sitting there and solving a bunch of them would be easier than botting the game enough.
This is, of course, circumstantial. It would not be appropriate for a simple "comment on a blog" kind of CAPTCHA, but it is exceptionally powerful in my case.