Mollom is pretty decent...
http://mollom.com works with a lot of cms's
It is commercial with 100 free positives per day, and can require captcha,
but it tries to avoid it with a crowd sourced algorithm approach



On 10/24/11 9:26 AM, "Ken Irwin" <[log in to unmask]> wrote:

>Hi folks,
>
>Some of our online forms (contact, archives request, etc.) have been
>getting a bunch of spam lately. I have heretofore avoided using any of
>those obnoxious Captcha things and would rather not start now. (I
>personally loathe them and they keep getting harder, which tells me that
>the spambots are probably better at them than we are...)
>
>Does anyone have some good/easy/free/less-stressful spam-inhibiting ideas?
>
>One that occurs to me to try, and I have no idea if this would match well
>with actual bot behavior: at the time the form loads, include at hidden
>field with id=[unixtimestamp]. When the form is submitted, ignore any
>forms that took less than (10? 15? 20 seconds?) to fill out on the
>assumption that bots probably do it way faster - or possibly way slower?
>Do they save them up for later? Should I add an upper bound? Is this just
>a really dumb idea?
>
>If I try that one, I would start not by eliminating the bad results but
>by marking them as spam and seeing how effective it is.
>
>Other ideas? (PHP-friendly answers would be easiest for me to implement,
>but others may work too.)
>
>What works for you?
>
>Thanks
>Ken