A security control

May sites have an image control on the registration form.  The image presents a blurry image of letters and numbers and protects against robot account creations.
What are these types of controls called?  Can you point me towards an example for MVC?
Thank you.
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Neil RussellConnect With a Mentor Technical Development LeadCommented:
They are called CAPTCHA's and MVC version can be found here on CodePlex

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