SDK

You can add new captcha types, import them from an ancient program or improve existing once. Unfortunately it is not as trivial as it might sound. You have to take care of some things here.

1. Filters and what they do
2. The right size of sample captchas
3. What method to use for what captchas

Some good tutorials have been written by very active forum user Ozz (thanks again):

SDK for Dummies - How to improve existing captcha definitions
SDK for Dummies - How to make your own captcha definition
A good tutorial that covers everything