Many popular Websites use captchas to prevent automated signups. The idea is to have a test which only humans can answer. Almost all captchas used now are image based. The user is asked to type in a word displayed on the screen. These days many of these sites offer alternate mechanisms such as audio for people with visual disability.

This week I was on vacation and I thought I will just go through the captchas of most popular Websites. What I found was a wide range of visual captchas. Some of these are difficult even for a human! These are listed in alphabetical order.

The following sites were selected from the list of Top 500 Websites by Alexa.

alibaba.com - This is a simple captcha and I think it will be easy to write a program to decode this!

bebo.com - Another simple captcha with plain background. Note that some letters are incomplete, but still readable.

craigslist.com - A captcha easy for humans, difficult for programs.

del.icio.us - Simple captcha with dizzying background.

digg.com - This is a tough one even for humans. Well, a lot of guys will be trying to break this!

facebook.com - Can it be more difficult than this? Tough for humans and machines.

fotolog.com - Note how the background makes automatic detection almost impossible. Good one.

friendster.com - Huh? Fun even in captchas? Nice one.

gmail.com - The captcha looks very simple. The trick is that they use different fonts and styles for every pageload!

icq.com - Interesting background! Note that some letters are grouped while others are not.

livejournal.com - Another tough one, even for humans! Note that typing in ‘Audio’ enables an audio test!

lycos.com - What the? can you read it? They even have their site name on the captcha!

metacafe.com - Ugly, but very effective.

live.com - Typical microsoft stuff. Lot of bells and whistles, but difficult to read!

netlog.com - Note how the thickness variation makes it difficult even for humans.

piratebay.org - Even the pirates have got a nice captcha with a threatening black background.

reddit.com - Another difficult captcha. Sometime almost impossible to read.

rediff.com - A simple captcha which is easier to read, but I guess can be cracked with code.

xanga.com - Clever one with random lines connecting letters.

yahoo.com - Another captcha using the random lines connecting letters.