1. CPNRB – Celtic Personal Names of Roman Britain.
The database of all Briton names confirmed in sources and found in inscriptions in the Roman period, from 1st to 5th century AD. Divided by period, location, tribe. Invaluable for coming up with real-sounding secondary characters.
2. DARMC – Digital Atlas of Roman and Medieval Civilizations.
This one has everything. Roman roads, settlements – named and unnamed, bridges, passes, temples, fortresses, villas… the most comprehensive map of Ancient Rome on the internet.
3. ORBIS – Stanford Geospatial Network Model of the Roman World.
Calculator of distances and travel times for the Roman Empire. Google Maps for Ancient Rome, using main roads and sea routes.
4. PASE – Prosopograhy of Anglo-Saxon England.
Similar to 1., a database of names but this time for the Anglo-Saxons. Covers all of Middle Ages, divided by locations, periods, occupations and more.
5. Rural Settlement of Roman Britain. Another detailed map of every archaeological find from Roman Britain.
An even more detailed map of Roman archaelogy than 2., but dedicated solely to Britain, rather than all of Empire. Down to single coin finds.