Map viewers are 3rd-party programs that analyze saved maps and display the contents graphically. They are useful in finding Underground Jungles and Floating Islands.
Currently-developed viewers
Terraria Mapper
By retep998: [LINK DOWN] Original mapping program, forum thread link broken -- found here: [1] Must be unzipped into the folder where Terraria keeps its world saves, usually Documents/My Games/Terraria/Worlds. Then click and drag the map file onto the executable.
Terraria Map Viewer
By UberJumper: [2] Basic Mapping Program with symbols denoting veins of gems, altars, Crystal Hearts, Chests, and Shadow Orbs
Terrafirma
By mrkite: http://www.terrariaonline.com/threads/terrafirma-interactive-terraria-mapping.10955/ Displays the output map in a scrollable window.