If you're going to do just one thing in your life, you're best off doing it really, really, well. You might even be rewarded for it if it's something other people like. JPEGmini ($20, free feature-limited demo) does one thing—further compresses JPEG pictures to save you disk space—and does it quite well. If you're a photo hound, you'll like it, and you might want to reward the company so you can use it. See all graphic design software reviews.
JPEG is a compressed format, but the amount of compression applied is variable and optional, ranging from slightly to extremely lossy. Most camera devices use a very mild amount of compression, producing relatively detailed images that are much smaller than uncompressed (raw) files, but still much larger than they need to be for general use. This is generally wise, as you can't restore detail that's been compressed away, but for most purposes, you can squash JPEG files quite a bit further before you see obvious compression artifacts. See all software reviews.
JPEGmini takes advantage of the underzealous compression habits of your camera devices to further compact your JPEG files (no other types are supported) while leaving them nearly as detailed. It trimmed my 234.5MB test folder of photos from a RADAR 4G phone and Canon 230SX camera to a mere 134MB, a very nice reduction in space. At 100% zoom, it was very difficult to tell the difference between the original and recompressed photos. Zooming in to the pixelated level, the differences still weren't something to stress over.
I created two copies of the photos involved for comparison purposes, as JPEGmini compresses the originals and doesn't create backups. Be sure you don't want the originals of anything you use it on. Also, once photos have been re-compressed using JPEGmini, that's it: You can't reduce them further by running them through the program again. This is as it should be when a program's purpose is to save disk space while minimizing loss of photo detail.
In this photo-crazed world we live in, JPEGmini takes admirable care of a simple need: Reducing the amount of space required by your visual memorabilia. It does it quickly, and it does it well using a simple, but elegant interface. I'd like it better at $10, but I like everything better at $10. It's worth giving the demo version a try. JPEGmini is also available for OS X. See also: Professional editing software reviews.