Nobody's going to argue that JPEG is the perfect image format. It can be severely lacking in quality, especially with higher levels of compression used. However, the small sizes it creates and the ubiquity of the format makes it the de facto for viewing photos, both online and off. A new group has promised an overhaul of the JPEG, offering similar quality levels, but with massively reduced file sizes. Dubbed JPEGmini the sample images they're providing offer a five-fold decrease in size.
The JPEGmini's website provides a rather gorgeous little tool to compare their compression to the default, but would be easy enough to fake. So to please all of us pixel-peepers, you can download the original and resized versions of the images, or try it on your own files. From our own looking at the comparisons, you'll be very hard pressed to find any quality differences, even at 100% zoom. For now there are no limits on how many files you can convert, but you'll need a free account to run image batches through the service.
The way JPEGmini works is by scanning the image using a quality detector that mimics the way that humans see images, and then it compresses in ways that won't create visible artifacts. By calculating how much compression they can use without visually warping the image, JPEGmini can significantly reduce file sizes.
The best part is that the resulting files are completely JPEG compatible, so they can be viewed on just about any device on the planet.
There are some diminishing returns with JPEGmini. The larger the image, the greater the advantage, with shots 8MP and up receiving the most compression, and those at VGA resolution only shaving off around 10% of their size.