Here is a great concept for a CAPTCHA.

The ROTCAPTCHA (PDF, 1.4MB).

It’s a CAPTCHA based on image orientation.

The ROTCAPTCHA system requires users to adjust randomly rotated images to their upright position. This is a task most people will be familiar with given the state of early digital cameras, etc.

This system has many advantages, such as being language independent and support many input sources.

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