All the ends and bits of vegetables, including onion skins, etc., are the most nutritious parts of the vegetable and lend flavor to the stock.  It's great if you can save them and add them in. 

As for eggshells, it's some of the most bio-available form of calcium (add a little vinegar when boiling), very similar to our own composition.  I haven't tried it yet, but will be saving my shells from now on.  It always felt a little weird to throw them out, and am excited now that I can stick it in the freezer collection bag.