mybb-captchapack is a clone of the Drupal module CAPTCHA Pack for MyBB — or at least, tries to be one.
./inc directory to the root directory of your MyBB installation.
mybb-captchapack could be considered of alpha quality right now, since I know pretty little about PHP, MySQL, and MyBB, and the tests I’ve done are few. Usage on productive sites is highly discouraged. For all possible outcomes of using the plugin (including but not limited to burning your CPU, making your monitor explode, or attracting aliens to abduct you) I’m not liable.
This plugin has only been tested on MySQL. I do not know whether it works on other database engines.
These CAPTCHAs are in design much easier to break than MyBB default image CAPTCHA or reCAPTCHA. They are good only because they are rarely used.
GPL-2+? I copied some code from the original Drupal CAPTCHA Pack module, distributed under GPL-2, notably the ASCII art character code.