TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
Unity - The multiplatform game creation tools for everyone.
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.
Godot Engine - Feature-packed 2D and 3D open source game engine.
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.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.