WesleyC - 3:33 pm on Jan 11, 2011 (gmt 0)
Not trying to toot my own horn or anything like that (and since this is an internal link I hope the admins don't mind), but this may be legitimately useful to any of you exploring alternatives to CAPTCHAs. I posted the CAPTCHA-less solution I use fairly recently on this very forum. While not perfect, it does a pretty darned good job wherever I've used it in the past, mostly in low-traffic or moderate-traffic sites that got a large number of spam submissions.
Spamblocking without a CAPTCHA [webmasterworld.com]