Like many of you, I am a big fan of a concise, well organized and easy to understand reference guide. Did you know that MIT has one for all things metadata? The MIT Metadata Reference Guide is a wiki with lots of information on standards, mapping, terms and uses of metadata.
There are project examples and reading recommendations and even some nifty crosswalks. While many of the project examples and readings are a little older, the content is both valuable, relavent and remains current. This would be a great starting point for students trying to gain a basic understanding of metadata or for a librarian that needs a refresher.