I have a server side check that helps to reduce spam by assigning a unique ID to each form downloaded and having that ID entered into the database along with a pointer to the form handler. Each form ID can only be used once, and if a form requires such an ID and no ID is present - then no form handler is called for. (This also helps to secure access to the system through other forms such as administrative functions.)
I understand there are more checks I could use server side and would love to hear about those, but I also want to focus right now on the client side. Do folks have any suggestions for captcha-less forms that are resistant to spamming?