the grails-app/utils/
directory is where items like Codecs go.
I'm not sure what else goes there, but you can include other classes there and they will be included within the Application
EDIT:
The grails-app/utils/
directory is meant for "Grails specific utilities" (Artefacts, GrailsClasses, etc.)
Jeff Brown(Core member of the Grails development team) responded to a question on the mailing list that confirms this. He said:
You can put just about anything you
like in the utils
directory but it is
really for Grails specific utilities.
At the moment I think the only thing
really targeted at this directory are
custom codecs. Your application
classes that are not Grails artifacts
should go under src/groovy/
You can find an example of a new Grails Artefact in the Constriants plugin. The Author of this plugin recommends that users put their Constraint classes in the grails-app/utils/
directory.