The people at Mozilla have produced a transcoding JPEG optimizer.
This thing is sweet: it takes ordinary JPEG files and recompresses them much more efficiently without making the images deteriorate. It uses the long-standing and well-tested Independent JPEG Group reference implementation, and adds a bunch of optimizations.
It’s not built for GUI users; it’s a command-line utility.
For those who publish a lot of JPEG images online, this could be a big time-saver for your audience and bandwidth-saver for your web server.