The Holy Roman Empire
Established in 800 AD (or, depending on your definition, 962 AD), the Holy Roman Empire dominated central Europe until its dissolution in 1806, covering the lands now known as Germany.
Taking the form of an elongated crib-sheet, this page contains a lot of detail about the empire, including definitions, titles, political constitution, and historical changes. If you’re looking for a narrative then this isn’t suitable, but otherwise the page is a valuable reference source.
A splendid introduction to the final centuries of the Holy Roman Empire by the University of Calgary, with some nice graphics, a hyperlinked glossary, and a political map of Europe in 1490.