I cannot conceive of reading or writing history – or teaching it – without maps. History takes place in particular places in time and the place characteristics do matter.
Perhaps the most accessible good quality maps are found in the Atlas collection at the History Department at West Point:
The Civil War Trust has a large collection of good maps, many animated, and worth checking out.
civilwaranimated.com has really nice animations of many Civil War battles and campaigns (related pages cover World War II)
The University of South Florida had an interesting collection:
Many of the maps used in Wikipedia articles are created by Hal Jesperson and many are available here:
For those who like or love topographic maps, MyTopo has a collection of historical topographic maps:
The Perry-Castañeda Library at the University of Texas has a fine historical Map Collection at:
The Hargrett Rare Book and Manuscript Library at the University of Georgia has a nice collection online at:
If you discover a good map collection that isn't listed here, please let me know.