The problem is that reCaptcha (a) used as a service, would pass private user data to a third party (b) is closed source, so we can' t just put up our own instance. Has anyone reimplemented it or any of it? There's piles of stuff on Wikisource we could feed it, for example.
OK, then we can take KCaptcha and integrate it as an extension. It's russian project, I've used it many times. Seems to be rather strong. http://www.captcha.ru/en/kcaptcha/