The United States Geological Survey has a great online mapping feature that is quite addicting and can suck you in for a while.
I recently discovered a very nice collection of antique maps right here on the web. The University of Alabama has an archive of old maps dating to the founding of our country. It is broken down by state and all the maps are high-resolution and readable.
Warning… be prepared to waste an incredible amount of time on this site! It’s very interesting and quite addicting for historic map lovers.
You can check out these maps by visiting this link.