<p>Besides the sample applications featured in the books <em>Beginning Groovy and Grails</em> and <em>The Definitive Guide to Grails</em>, are there any applications out there written in Grails that are good examples from which to learn best practices? Such as real applications that are open-source? Thanks.</p>

Yes.  IBM has an excellent series of articles building on to a sample application at  IBM's developerworks site:
Part I:
http://www.ibm.com/developerworks/java/library/j-grails01158/
The Series:
http://www.ibm.com/developerworks/views/java/libraryview.jsp?search_by=mastering+grails