TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
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.
Invision - Prototyping and collaboration for design teams
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.
Sketch - Professional digital design for Mac.