What would you do with a magnetic monopole? I might make this an interview question at work. Hmmm
Make an infinite energy machine by having magnets on a rotor, surrounded by monopoles on a stator (or vice versa), so the rotor is infinitely accelerated with no external input energy. Hook it up to an alternator and harness the free power.
I'm pretty sure that doesn't work for the same reason that replacing monopoles with charged objects (which we can actually build!) doesn't produce a perpetual motion machine.