Saw this on National Geographic's website and was instantly blown away. Each day, they post a high-resolution historical map along with a brief background and a trivia question about the area in question. They have a very nifty zoom tool that allows you to change the magnification and pan around the map and the image quality is startling.
Today's map which you can see here is the original plan for the city of Annapolis, Maryland dating back to 1718. It's in honor of the anniversary of Maryland's founding which is this weekend.
If you have even a passing interest in old, historic maps, then you'll definitely enjoy this. There's a different map each day (hence the title) and you can browse all the maps posted from earlier in 2008.