Leshy SpriteSheet Animator
is an HTML5 tool for previewing and testing sprite sheet animations. For documentation, see the introduction blog post.
A follow up blog post
discusses the file formats and support for exporting to CreateJS/EaselJS and ImpactJS. To add sprite sheets (PNG) or load save files (JSON,) drag and drop the files onto this window. (Browser Requirements: Recent versions of Chrome, Firefox, or Opera. IE is untested. Chrome is recommended.)
(v1.0.1, updated 6/16/13)