The proof-of-concept application I use whenever I test any development framework.  It is trivial yet the domain model contains embedded objects, one-to-one, one-to-many and many-to-many relationships. So it lets me flex most framework features.

In this version, it is a Maven based Groovy project using Hibernate JPA.

Groovy 1.7.0
Hibernate 3.5.1-Final

I'll slap a web front end on it when I get a chance.