Edit JFIF Comment is a small application that lets you view and modify the textual comments included in JFIF files (more commonly known as JPEG files). Doing so doesn't modify the compressed image data, which means that the tool is quick and lossless. Besides, it is scriptable, so it can be used to modify many files at once.
I needed such a tool but couldn't find one, so I wrote it!
It's freeware and runs under MacOS 7 to 9. Here is Edit JFIF Comment 1.0.4 (120K).