Really Simple CAPTCHA
See captcha code below
<p>Your Name (required)<br />
[text* your-name] </p>
<p>Your Email (required)<br />
[email* your-email] </p>
<p>Subject<br />
[text your-subject] </p>
<p>Your Message<br />
[textarea your-message] </p>
[captchac captcha-118 size:l fg:#00ff00 bg:#ffffff]
[captchar captcha-118 4/4]
<p>[submit “Send”]</p>