Since the CAPTCHA on the Feedback form seems to have done a good job on stopping spam (yay!) I've also now added them to the Email Contact Form includelet and to Surveys.
For the Email Contact Form and Survey includelets, there is now an option to turn on the CAPTCHA, which will be displayed to anonymous users if enabled.
From within the Surveys app itself, if you make a survey Public then it will display the CAPTCHA to anonymous users.
In all cases, logged in users will not need to complete the CAPTCHA in order to submit the form/survey.
Enjoy, in a spam-free way!