I built this tool two years ago because I didn't have means to get Texture Packer. Now I have decided to release it to the community: https://umesh-kc.itch.io/free-texture-atlas-generator-web-app

It allows you to export atlas image in PNG format and atlas data as a simple JSON file which you can integrate with your game engine easily.

Although it uses a different packing algorithm than Texture Packer, the packing efficiency is still pretty good. You can experiment with different sorting functions which are selectable from the drop-down in order to see the most optimal one  for your set of input textures.