JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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There are several captchas that you can add to your JotForm.
You can see 3 of them in action on JotForm here: http://form.jotformpro.com/form/42593404965967
Now you can use the link and clone them to your own JotForm, but to add them yourself to the JotForm you can find them here:
Go to quick tools
Now you will see captcha widget, so you can add it to your JotForm
If you want to find some more interesting or unique captchas then you can click on the Widgets (right under the Quick tools and in the same menu) and search for captcha
This is how they look on the form to your users:
If you have any trouble adding the captcha widgets to your JotForm do take a look at this easy to follow tutorial here: http://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form
I hope that this helps you and do let us know if we can help you with anything else.