I had a client who was unhappy with the reCAPTCHA included on a contact form. I replaced it with an alternative which I told him probably wouldn’t be as effective, but would at least be invisible. I added a ‘honeypot’ feature - where the submitted form fails validation if a particular field is given a value.
I gave the field a plausible name and ‘hid’ it with css:
#preferred {
position:fixed;
left: 2000px;
}
(probably more reliable than visibility:none)
I appreciate this solution is not as sophisticated as a reCAPTCHA but it has worked well in this case. I added the ‘honeypot’ about a year and a half ago. I redirected bad submissions (where the hidden was given a value) to myself - there have been almost 1400 so far. As far as I know, my client has not received a single email from a robot during this time.