PixiEditor (App)
PixiEditor is a free, open source 2D graphics editor that you can use to create video game sprites, animations, pixel art, painting, and even vector art.
Posted in2D Art Resource Free Level Editor Open Source
Tiled (App)
Tiled is a free and open source level and map editor designed to help game artists and developers create tile maps for 2D games.
Aseprite (App)
Aseprite is an open source pixel art tool designed for creating animated sprites and pixel art.