TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
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.
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.
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.
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.