Free GM Resource: Mappery
There are a lot of places to find maps, but fewer places to find collections of more modern maps (maybe with a few older ones sprinkled in). There are enough collections here and there (I'm sure I've highlighted some in the past), but this one interwebs source, mappery, is pretty cool.
In addition to the requisite map collections there are a few great features worth checking out. One is the ability to have your own "room" where you basically assemble your own collection of maps. The best, in my opinion, is a "Gallery View" where you can pan-in on a big world map and see where the maps themselves are located. You can jump straight to the world view, but it works better if your narrow your focus first and then switch over. If there are too many maps in a small area they don't show up on the global view.
I went to the Antarctica page and then switched to the map viewer and got to see the location of the maps in the collection. Pretty cool, yes?
Next time you need access to a real-world map, you should check out mappery.