Aseprite (previously ASE, aka Allegro Sprite Editor) is a program to create animated sprites & pixel art. Sprites are little images that can be used in your website or in a video game. You can draw characters with movement, intros, textures, patterns, backgrounds, logos, color palettes, isometric levels, and more.
Aseprite source code is freely available from GitHub but pre-built binaries are paid software.
You can download the source code, compile it, and use it for personal/commercial purposes, but you cannot redistribute Aseprite to third parties.
The paid version, $14.99/USD, includes pre-built binaries for Windows, macOS, and Linux, as well as updates and priority support. The trial version does not allow you save your work.