uCaptcha is designed for people to customize their own captcha and render it on the fly, it creates a value, renders that in the captcha image and also saves the value to a session variable called 'captchaString' so you can check against it.
uCaptcha is distributed under the MIT license which means it can be used for free and distributed in other works however you must provide acknowledgement in the credits.
For assistance with uCaptcha please post on the our.umbraco.org forums in the uCaptcha help section.
1.2 - Added ability to change font color using RGB values in the uCaptcha config file.
1.1 - Added support for custom fonts in config file and fixed error where some systems would disallow the session variable causing a null reference error.