Compress JPG, GIF & PNG Images – punypng
punypng is a free web-based application which can seriously compress JPG, GIF & PNG images without any loss of quality.
The application first analyzes the content of your image and figures out the best image format to use. If necessary, converts the format automatically.
It also has built-in support for dirty transparency* which can save an average of extra 10% for transparent GIF & PNG files.
What would be more amazing is the service to offer a desktop tool with context menu support.
*: an innovative technique that offers additional compression to any PNG or GIF as long as it has either 8-bit or 1-bit transparency