gimp ist your friend, man..
What's wrong with you people?
I do not lack tools, I'm annoyed at the inability of orientation metadata to work in a portable fashion.
Besides, using Gimp (which is a nice program, if you need that kind of editing power) to flip a JPEG 90° is like shooting fish in a barrel with a Gatling gun. Total overkill. (which I defined as "appropriate" upthread, I know!)
A much more appropriate solution is to use
convert from the ImageMagick suite, like so:
convert input.jpeg -rotate 90 output.jpegWith a small matter of programming you can flip an entire album if you so wish, without opening every file by hand.