WP-Biz (Ryan)

The “honeypot” technique for SPAM protection is invisible to humans, and tricks robots into filling out an invisible form field. Then, if that invisible field has been filled, the form cannot be submitted. Easy, non-instrusive SPAM protection.
No settings to set, this honeypot is added to all Formidable forms on your site, unless the user is logged in. When SPAM is suspected, the form will not be submitted and the visitor will be alerted to this fact.
Tested to work with both Formidable (free) and <a href="http://www.wpbiz.co/go/forms" rel="nofollow ugc">Formidable Pro</a> (best bet for awesome form power!)
A note to Formidable Pro users: You paid to receive extra features in the Formidable plugin. Me too, it’s well worth it. This Honeypot plugin, and Ryan (the developer) are not connected to Formidable in any way. Since this adds another very helpful feature to Formidable, I ask you to send a <a href="http://wpbiz.co/donate/" rel="nofollow ugc">donation at WPbiz.co</a> to say thanks and encourage future developemnt.

Formidable Honeypot

Add SPAM protection to your Formidable forms without annoying your visitors with Captchas, math, or other fields.

thomstark

A lightweight Formidable Pro anti-spam add-on. No intrusion on the user, simply checks the time it takes to submit the form and generates an error when that time is under a certain threshold.
<h4>Requirements</h4>
<ul>
<li>Formidable Pro 1.7+</li>
<li>WordPress 3.7+</li>
<li>PHP 5.2+</li>
</ul>

Formidable Anti-Spam

A lightweight Formidable Pro anti-spam add-on that requires no interaction with the user.

Formidable Honeypot vs Formidable Anti-Spam