Recently heard about this strange class of microbes, magnetotactic bacteria, which produce and contain magnetosomes. These are iron complexed with protein and are arranged intracellularly as a chain. The iron is in the form of magnetite, the same form present in naturally occurring lodestone. Magnetite, as you might have guessed, is magnetic. The magnetosomes in the magnetotactic bacteria facilitates magnetotaxis ie. their movement based upon the magnetic field of their environment. And this, it is thought, is the purpose of these structures in the magnetotactic bacteria. These bacteria are very senstive to the redox potential of their environment and use the magnetic field of the earth in order to find "down" to a less oxygen rich environment. Thus bacteria in the northern hemisphere have their magnetosomes arranged in such a way as to get them to swim to magnetic north, which is slightly down in the northern hemisphere, away from oxygen. The opposite it was thought was true in the southern hemisphere. I ran into a great science article that sheds some doubt as to this purpose of magnetosomes. Apparently this group found "south-seeking" magnetotactic bacteria in the northern hemisphere.
I'm just guessing here but when I first heard about these bacteria I thought that the magnetic field they produced might have been useful for biofilms or to arrange themselves in some bacterial community. But that's a nature paper for someone else.
Magnetosomes are pretty interesting themselves and have some biotechnology applications.
What is actually pretty spooky is that not only do migrating birds and salmon have magentosomes, but so does the human brain.