non-captcha plugins please

[gpmossop]

Hi, I'm trying to design a membership site. When I registered my name on this site it didn't ask for the frustrating Captcha letters that none of us humans can read nor understand the verbal translation. Could someone please tell me where to get this type of non-captcha plugin (that this jdownloads.com site uses)?
Would it be called a human-questions-to-answer-only plugin? If I have got this wrong please correct me.
all ideas would be appreciated.
gm

[Rommel7]

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