This extension is a clone of the typo3 extension "captch" written by Kasper Skaarhoj. It has been modified in order to work with TYPO3 6.0. However, this extension is not backwards compatible to previous versions of TYPO3 (< 6.0).
Original credits go to https://github.com/cornelius-illi/captcha, I just made another bugfix to get it to work properly in my TYPO3 6.1 installation.