Starting a struts2 + spring + hibernate project from scratch and creating an image gallery out of it.
I don't know about you, but starting a brand new struts2 project was a pain in the butt. But now I've gone through it, I have some insight into how things work.
There is also another reason, and that has to do with object oriented sql. If I can get that working with Hibernate, then I've learned much more than some enterprise applications can teach me.