I've always loved maps. When I was a kid we had this Encyclopedia Britannica atlas that was simply beautiful, although the paper was so stiff it gave you wicked paper cuts. But on the web, you don't have to worry about paper cuts.
So if you've gotten tired of Mapquest or Google maps, it's time to move on to the maps at The Global Distribution of Poverty.
When you're done there, you can check out all the maps NASA's Hurricane Data Portal.
Maybe you would be interested in a map of early modern London. Maybe not.
If you are into real estate, how about Trulia or Zillow
Want to make your own map? Try Frappr.
Want to find out where all the historical markers are? Then use The Historical Marker Database, or course.
Or how about USGS?
Or how about the the Library of Congress Map Vault?
Or how about a site whose name says it all, Worldmap.org. If you can't find it there, can you find it anywhere? OK, it's concerned with Christian missions so you can find maps dealing with Christianity there.