Ancient and Medieval History Online is a database of information covering various civilizations, including: ancient Egypt, ancient Greece, ancient Rome, medieval Europe, ancient America, and medieval and ancient Asia, among others. The site includes biographies, events and topics, primary sources, timelines, images and videos, and maps and charts.
This seems to be one of the best online resources I’ve come across to date. I especially love the medieval primary sources (the Anglo-Saxon Chronicle, the Capitulary of Charlemagne, the Magna Carta, etc.) and the medieval maps and charts (the Anglo-Saxon invasion of England, the Black Death in Europe, the Division of the Carolingian Empire, the Kingdom of France, the Lands of the Picts, etc.).