Several months ago I threatened to make the captcha generation I use on this site available as a web service. I have finally done that, at captchaservice.org. I was able to do this in part because CommerceNet was kind enough to donate and host the server that captchaservice.org runs on. Thanks CommerceNet!
In addition to providing the kind of distorted text images that you see here when you add a comment, I have a couple of pure-text captcha types as well. The motivation here was pure accessibility-guilt, and the captchas are extremely lame and crackable in comparison to the image-based ones. Devising automatically-generated pure-text captchas that are difficult for programs to solve looks like a hard problem; in particular, any text you generate with a tree structure can be correspondingly parsed. I’m hoping that someone at this workshop has advanced the state of that art.
I also added a new type of image-obfuscation that looks like this:
Here’s a sampling of the captcha types that captchaservice.org supports so far.