Simple Recaptcha validation HTML form fragment when using Google App Engine (GAE), Webapp2 and WTForms.

recaptcha.html

<label>Verify you're human:</label> 
<script type="text/javascript" src="http://www.google.com/recaptcha/api/challenge?k=YOUR_RECAPTCH_PUBLIC_KEY_HERE">
</script>
<noscript>
  <iframe src="http://www.google.com/recaptcha/api/noscript?k=YOUR_RECAPTCH_PUBLIC_KEY_HERE" height="300" width="500" frameborder="0"></iframe><br>
  <textarea name="recaptcha_challenge_field" rows="3" cols="40">
  </textarea>
 <input type="hidden" name="recaptcha_response_field" value="manual_challenge">
</noscript>

Here's the controller code for a Google App Engine, WebApp2, WTForms application:

https://gist.github.com/mengelhart/8045030