A data validation framework to be used with data models. You just include Vandrake into your class, and it gives you methods to construct a validation chain for the model attributes.
As of right now, all the popular Ruby ORM/ODM frameworks are based on ActiveModel::Validations in some form or another. While it's an excellent validation framework, I felt that the simplified approach for reporting validation errors wasn't a good fit for things I was working on.
Instead, I decided to port the custom validation framework from an API project I was working on. It is a battle-tested system, with more detailed (and machine-parsable) errors, and full introspection support for generating documentation. Plus, thanks to Ruby, it now has a nice clean DSL slapped on top of it.