I just noticed that RapidShare had implemented a CAPTCHA with cats and dogs (I haven’t been to RapidShare.COM for a while). Their original one wasn’t that hard to break. The new is interesting because it requires a human to recognize the animal that is shown. Computers can’t do that.
But it looks to be still weak. It appears that Rapidshare uses the same image for the cat and dog for every CAPTCHA. It would be easy for a program to find the cat and dog images on the CAPTCHA, extract them, identify them, and remove them from the image so that the text can be very easily OCRed. Maybe if RapidShare distorted the image, used different fonts, added random lines, and used different images for the animals, then they would have a real chance against those automated download programs. In fact, I wouldn’t be surprised if it already has been broken.
Edit: Well, (unfortunately…) it appears they have now added distortion and blurring… >.>