Steinar H. Gunderson <email@example.com> has added JPEG compression previewing to GIMP. As of 1999-08-21, it's only available through the CVS version of GIMP, but it should become available in GIMP 1.1.9 when it's released.
For those of you without CVS GIMP, I've taken a couple screenshots of the previewing in action. They're on the big side, but they give a good idea of how it works.
Screenshot 1 (252K)
Screenshot 2 (247K)
Original PNG (313K)