Several independent works are being distributed along with Tiled that fall
under different licenses. You can find the license that applies to each file
in the comment at the top, but for clarity they are also listed here:

Software        Directory                 License
Tiled           src/tiled                 GPL
libtiled        src/libtiled              Simplified BSD license
tmxviewer       src/tmxviewer             Simplified BSD license
libtiled-java   util/java/libtiled-java   Simplified BSD license
tmxviewer-java  util/java/tmxviewer-java  Simplified BSD license

The full text of each license is provided in the files LICENSE.GPL