TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.

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.

Stitches - A template generator with functional CSS

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.

Sprite Sheet Animator - Have a sprite that you want to show off?

Sprite Monkey - Small and fast but still full-featured sprite sheet editor.