Radiant-cpatcha-tag is a project mainly written in Ruby, it's free.
radinat cpatacha tag extention
= Custom Tags
= Very basic and simple cpatcha that can be used in forms.
=
CPATCHA that are used: http://captchator.com/
How to install extention: https://github.com/radiant/radiant/wiki/Installing-Extensions#wiki-manual
Custom tags : /app/models/custom_tags.rb
Using with Mailer extention: git://gist.github.com/1226154.git
I think that this is not very good solution but I'm not familiar with Radiant