One of the challenges I have with reading histories, especially older ones, is that i'm to some extent a visual learner and what i'm reading will mention cities, places and geographical boundaries that are no longer familiar in the modern world. Sadly most of the accounts don't offer their own maps of say, Europe in the 1500s or the extent of the Byzantine Empire in 983, so i'm often left out in the cold.
Is there any kind of online resource that maintains this data for easy reference? I feel like I often need to have a general map on hand as I study given period of history to refer to in order to keep everything clear in my head.
I don't know of a good one-stop online source for maps, other than Wikipedia, but there is a really nice little book that I own called The Penguin Atlas of Medieval History that might be along the lines of what you are looking for. There is a newer version out that updates the maps (since it's an older text), but when I want to get a grand overview of what Europe looked like around a certain time period from c. 300 to c. 1500, that is the book I pull out. There are also other Penguin atlases that overview other eras too.
For really detailed maps though, you sometimes have to go digging in texts on specific eras. Warren Treadgold's A History of Byzantine State and Society has some nice maps of various periods in Byzantine history, as does The Oxford Dictionary of Byzantium. I have some really old medieval atlas books with the epic old style maps, but I doubt they are easy to acquire, since they're like 50+ years old and probably out of print.
This should be very helpful :
http://geography.about.com/od/historicmaps/
and this is also a great source http://maps.nationalgeographic.com/maps