The tiled map editor
Tiled is a general-purpose tile map editor. It is meant to be used for editing maps of any tile-based game, be it an RPG similar to what we saw in Chapter 6, JRPG – Second to Last Fantasy, or any other type of game. It is mostly written in C++ using the Qt libraries, which means that it is compatible on most platforms. It can be downloaded for free from their official website at http://www.mapeditor.org.
The source code is completely open source and is under two different licenses (GPL and Simplified BSD depending on what code you are modifying) and can be found in GitHub at https://github.com/bjorn/tiled.