edjpgcom is a free Windows application that allows you to change (or add) a JPEG commment in a JPEG file. That's all it does. All other fields in a JFIF or Exif file are left untouched. It even keeps the filesystem timestamp! It's based on the rdjpgcom and wrjpgcom utilities from the Independent JPEG Group's 6b distribution. (Heck, it's essentially these two programs combined with a basic dialog control.)