Reverse-engineering proprietary magstripe cards
Many years ago in college I was curious to see if I could hack the meal cards we were issued.  These were plastic like credit cards with a mag stripe.  You swiped them thru a reader instead of paying cash in the cafeteria.

After finding the right formula, it turned out to be surprisingly easy to see the magnetic bits and figure out the encoding.  To see the bits, I filed off a bit of the ferrite rod antenna of a old dead radio.  The filings clumped at the magnetic transitions.  This was made easier by suspending the filings in a soap solution.  The liquid allowed the filings to move around more freely, and the soap kept the liquid from balling up into individual drops.

This was over 30 years ago, but even at the time I was suprised by the low data density.  It turned out the whole card only held something like 36 bits.  If I remember right, it had two tracks, one clock and on data.  Each group of 4 bits represented a decimal number, and the last number was used as a checksum.  Yes, it really was that primitive.

36 bits spread out over maybe 2 inches were actually quite easy to see with the naked eye.  We could decode someone's card in a minute or so.  However, attempts to make our own cards failed.  The ferrite filings only show you magnetic transistions, but not polartiy.  I had a two-track tape head that I rigged up to get reasonable voltage changes as I manually moved it accross a stripe, but with only a analog scope and nothing like a microcontroller to capture the information, I couldn't really see enough details of the signal to duplicate the magnetic pattern properly.  Nowadays with digital oscilloscopes, microcontrollers, and logic analyzers, that would be much easier to do.  And yes, I had to walk to class barefoot in the snow uphill both ways too.