I use a double pronged approach on some of my forms.
I have a field that must remain blank (or else it returns a message saying that we suspect you may be a machine). You have to be careful what you name those remain blank fields for a couple of reasons: 1) Google Toolbar autofill - it might stick a value in there unbeknownst to you and 2) It has to make sense to someone who might be seeing the field with a screen reader (maybe that's not an issue if you set the visibility off).
Secondarily I put a hidden field in my form that is populated when the onclick event fires on the submit button. I then check for that form value on the server side to make sure it's populated with the correct value. This should prevent bots from posting to my page. It basically requires the human interaction of clicking on the button to pass validation.
If you're interested in some code let me know.. I was going to blog about this, but have not gotten around to it.
I have been using reCAPTCHA (http://recaptcha.net/) on some of our forms with success. I am sure it will only be a period of time before this is useless as well. It has an audio CAPTCHA built in for accessibility, and is fairly easy to implement.
Seconded. I have had great results on our blogs with reCAPTCHA. The only spam comments that have come through are obviously human. Before that they were getting hit so much I have to have registration on, which meant no one went through the effort required to comment.
The forms we were having problems with did not ask for any urls. So, I wrote a script that checks each item in the object. If any of them contain "http://" then the script stops and returns to the form page.
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