Is there a website where we can find out which grocery store has what
items on sale? Ideally it would be user generated, so anyone could
post what they see on sale at the local store.

This would probably be a cooler service if the grocery store
themselves made this information available on there website. I could
see Wal-Mart integrating their barcode scanners with the website. So
soon as they scan a new pricing for an item it would show up online on
the website. I don't see this is technically challenging to do, so not
sure why it has not been done before.