Add Checkbox to SPForm

Is there a simple way to add a checkbox to SPForm for someone to say, opt into or out of a newsletter? I know this is a somewhat subjective question. If there is a relatively simple way to do it, I'd love to hear about it.

SPForm is designed to be a quick-and-dirty contact form. It doesn't lend itself well to customization. You could add the checkbox to the Tpl chunk, but I don't think there's any way to process it.

FormIt would probably be a better choice if you want to customize the form.

You might also look at the Subscribe extra (with or without the Notify extra). Subscribe lets users express preferences when registering and provides a page for them to manage the preferences and unsubscribe. Notify will send an unsubscribe/manage preferences link with every email it sends.