During my learning phase, I came across several good sample applications.
This sample web album app got me started on grails. It covers security, image upload / manipulation, basic crud etc.
http://www.syger.it/Tutorials/GrailsWebAlbum.html
Here's one from Glen Smith, author of the upcoming Grails in Action book. The source code to the groovy blog aggregator is located here:
http://code.google.com/p/groovyblogs/
Here's another one by Glen, a blog engine written with grails:
http://code.google.com/p/gravl/
And finally, here's the code to the grails.org site which includes wiki functionality:
http://github.com/grails/grails/tree/master/grails-samples/grails.org/