mccarthy/phpFormProtect
phpFormProtect allows you to protect a form from spammers without using a captcha. It uses a number of different techniques to assess a given submission, and then scores the submission similar to email spam filters, but much simpler. It could certainly be overcome - but in our experience it provides all the protection needed for most sites. It is a port of CFFormProtect (http://cfformprotect.riaforge.org/) - many thanks to the folks there.