Tiled

You paint tilemaps in layers from a tileset, drop in objects such as spawn points and trigger zones, and save the lot as TMX (XML) or JSON. The editor knows nothing about your game, which is its strength: loaders exist for practically every engine and framework, and it serves platformers and isometric strategy games equally well.

Thorbjørn Lindeijer started the project in Java in the mid-2000s and rewrote it in C++ and Qt around 2008. It has been funded by its users rather than a company ever since. Rule-based autotiling is built in, and the younger LDtk is the nearest thing to a challenger.