Smushit is a service that goes beyond the limitations of Photoshop, Fireworks & Co. It uses image format specific non-lossy image optimization tools to squeeze the last bytes out of your images – without changing their look or visual quality. You’ll get a report of how many bytes you can save by optimizing your images and all the changed images as a single zip for download.

Smush it comes in different flavours:

• You can upload a bunch of pictures in your browser
• You can provide them with a list of image urls or
• You can get a Firefox Extension to optimize the images found on any web page