Captchas were an innovation designed to prevent abuse in the form of bots or other automated spam tactics. These days, many people believe that captcha is an unnecessary annoyance. This thought might not be true in all instances but it is certainly true if the captcha is illegible or is very difficult to solve.
One of the most commonly encountered captcha types are by Recaptcha. The second word is frequently unreadable but apparently it is designed to be that way. A user can just type the first word correctly to pass the captcha. The second word entered is reused by Recaptcha as an interesting way to generate more random verification phrases.
The set of captcha seen above are the more ridiculous type unless they are targeted specifically for people that can solve them. These critical thinking and math captchas do hold a measure of effectiveness against bots if they cannot be solved by simple formulas.
A hilarious attempt to harness human emotion against the automated spam. "What is love?" =)
I chuckled a bit at this one but it would be fairly effective against automatic scripts if the instructions to sketch the drawing were not recognized. Maybe if this captcha was not so time consuming, it can be for artists.
This captcha is one of the more effective ideas as bots generally won't be able to figure out the purpose of a picture. A captcha like this is fairly simple for a user to solve but the greatest weakness of this captcha is the lack of randomness. Categorized images are not as easy to randomize as letters and are likely to be more limited than phrases.
An experimental attempt to develop a powerful anti-bot captcha by Google. This method exploits the weakness of automated systems in recognizing images and figuring out instructions. It is highly unlikely that a bot or any attempts to create one will be able to figure out which image is correctly right side up.