We started getting spambot attempts through our Contact forms. None of the mail actually got through due to the nonexistent "from" addresses, but still, lots of junk mail with which I had to deal.
I installed OSOLCaptcha to try to handle the form-generated Spam. This extension works PERFECTLY, and with very little configuration. A little added CSS for styling and I was done. So far, we are down to about 1 spam attempt every few days; I suspect these are coming from humans.
Kudos to the developer for something simple, elegant and easy to implement!