Eihort is a
Eihort lets you choose any of the Minecraft worlds you have saved on your computer. You can then fly freely around them. An additional menu lets you toggle various views. You can view the Netherworld or Overworld, create new spawn points for your player and view the world from huge distances.
Perhaps the most useful tool in Eihort is it will highlight certain types of block for you, so you can locate them in the game easily. This means finding rare ore like diamond becomes much easier. The in-game like presentation means Eihort is also great for taking screenshots of your in game creations!
The only issue with Eihort is that the huge draw distances can mean it uses a lot of resources, and can be quite slow on computers with little RAM.
Eihort is probably the best Minecraft Map Viewer, and is certainly the easiest to use.