Free Texture Packer - Free Texture Packer is an open source tool that allows you to pack multiple images into one atlas for you games or sites.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Sprite Sheet Packer - Sprite Sheet Packer is online that you can easily use to pack sprite sheets, including data formats for CSS, LESS, and Phaser.
Synfig Studio - Just another WordPress site
Sprite Monkey - Small and fast but still full-featured sprite sheet editor.
Adobe Animate - Adobe Animate is a Flash, vector animation software.