Captcha work - Are they legit? How much can 1 earn from it?

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I'm a bit naive in this captcha thing...I'm just trying to earn a bit extra by working online...so wanted details on captcha work...are there any legit sites, which are for free registration and which are paid registration, how much can one earn (earning rate), etc., etc. Guys please throw some light....might be of help to others as well Thanks and happy mylotting.