At the beginning of this year I asked some similar questions. I read the book Head First Java which PhiLho recommended in that thread and it helped me a lot to understand the concepts better. Recently I wrote a concrete example for someone that uses both polymorphism and interfaces, see this thread.

Ah, thank you!  I recall seeing that post with PhiLho's response.  I think between that post and another one I read around the same time, I went and got Head First Design Patterns.  I like the book, but they do delve into using interfaces and other tricks that I understand conceptually, but haven't actually used in practice yet.  So, I'm trying to go back and fill in my skillset so I can advance through that really cool and useful book.

For what it is worth, I made a sketch using interfaces at http://bazaar.launchpad.net/~philho/+junk/Processing/files/head:/MazeGeneration/
It is simple enough to be understood, I think, and it might illustrate the interest of interfaces. Might be one of the concrete examples you are looking for.
Actually, I planned to add more to this sketch, but I never found the time...

I see that it's the same link as from the four months ago post that amnon.owed linked.