I think we all agree that we need -some- form a protection against spambots automatically signing in to gmail accounts, posting annoying links in webforms, etc. But man, there's got to be a limit. Captcha's are generally considered as a "bad user experience", and should be avoided as much as possible. These guys didn't take the hint.
And Rapidshare wins the prize for most unreadable captcha -- or am I just too stupid to see the difference between a cat and a dog?
Or let's distinguish colors. No wait, let's blur them out completely first, then let the user try to decipher them. That had to be the reasoning behind this one.
Or how about this one, used in a guestbook. Seriously, just close the damn guestbook -- no one will post in it.
And then there's the complete riddle. These are the kind of tasks you expect in a quiz, not to place a comment on a website.
The last one could be considered fun, if you have some free time and want some brain-puzzles to figure it out. I bet it gets a whole lot more annoying if you have to enter that one 10 times a day.
Time to come up with new -- and more userfriendly -- ways of keeping spambots away. Got any ideas? Javascript solutions seem to discriminate about 5-10% of all internet-users. Then again, discrimination seemed no fall-back for captcha's, so perhaps we should implement drastic solutions ...
Of your four examples, the one you labeled “worst ever” is the only one that I find acceptable. And I find it quite acceptable.
It obviously doesn’t suffer from random-distortion illegibility. It doesn’t present issues to color- or motion-limited users. It can be made vision-impaired accessible without too much trouble. There are minimal cultural assumptions (the use of = in the legend would count against it). And it pretty clearly satisfies the objective of excluding spam robots.
Legibility is a serious problem with captchas. I use a service that has added captchas with random distortion, background noise and case sensitivity, using a modern (aka grotesk) sans-serif font with minimal distinguishing detail between glyphs under the best of circumstances. I typically have to reload 4 or 5 times before I get one I can read with any confidence. This does not please me.
Other captcha options include showing 4 pictures of women, and let the user select the 2 most attractive ones (obviously, this would have to be really obvious, or you’d get in to the whole “I like [insert random name here] more!”). This is a good technique, if it’s used properly.
In my “worst ever” example the difference between a cat and a dog is hardly noticeable, you’d have to print it out and use a magnifying glass right next to it, just to see the difference. All you see a is a blurred animal, it might even be a giraffe. The logic behind it might work out, but it was put in practice very poorly by Rapidshare.