JPEGmini is a free-to-use online service for reduce the file size of your images/photos by up to 80% while keeping their original quality and JPEG format.
JPEGmini is a patent-pending photo recompression technology, which significantly reduces the size of photographs without affecting their perceptual quality. The technology works in the domain of baseline JPEG, resulting in files that are fully compatible with any browser, photo software or device that support the standard JPEG format.
The performance reaches the top when the images are high quality photosand edited-resized-processed images gets minor improvements (but still good).
Currently, it is available only as a web-based service (no command-line, API, etc.) and although it is possible to compress images with no sign-up, a free account comes with features like bulk upload and unlimited usage.
Demo: http://www.jpegmini.com/main/shrink_photo
Website: http://www.jpegmini.com/