Brandon Fenning

Since the vast majority of WordPress spam is automated in nature, the goal of BotBlocker is to block comment spam bots and not hinder users. This is accomplished through the use of honeypot technology, which tricks the spambot into identifying itself by performing actions that a human could not.
BotBlocker’s features include:
<ol>
<li>Works out of the box to block spam</li>
<li>Automatically stops spam-bots from commenting</li>
<li>Zero hindrance to users: no CAPTCHAS, math questions, passwords or extra input required</li>
<li>Zero false positives and ignores registered users</li>
<li>Can be configured to completely block spam-bots or flag comments as spam</li>
<li>Spam detection messages can be easily adjusted</li>
<li>No javascript or cookies required</li>
<li>Hooks into <code>wp-comments-post.php</code> to prevent spam-bots from directly submitting spam comments</li>
<li>Should work fine with most customized comment forms</li>
<li>No API keys required</li>
</ol>
http://www.lform.com/botblocker/ for more information.
Plugin by <a href="http://www.lform.com/" rel="nofollow ugc">LFORM, a web design company</a> in New Jersey.

BotBlocker

Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.

RedMoon

WPSForm

A lightweight addon for SimpleForm plugin. No configuration required. Once activated, you will find the new “Akismet Anti-Spam” option in the Settings page within the Spam Protection tab. Ensure that before you enable this option, you have activated Akismet plugin and registered an Akismet API Key for enabling anti-spam protection. You no longer need to worry about getting spam in your contact form, since each new form submission will be filtered for spam using Akismet.
For more detailed information refer to the <a href="https://wordpress.org/plugins/simpleform/" rel="ugc">SimpleForm</a> plugin page.
<h3>Demo</h3>
You don’t know SimpleForm yet? Check out our <a href="" rel="nofollow ugc">Demo</a> and find out how it works.

SimpleForm Akismet

Do you get junk emails through your contact form? This plugin for SimpleForm helps prevent spam submission.

BotBlocker vs SimpleForm Akismet