Magnets are awesome. Electromagnets are even cooler. But what if you could make a semi-permanent switchable magnet that acts like an electromagnet, but doesn’t use any energy to hold metal? You’re going to want to take a look at this Low-power Magnetic Hold and Release Mechanism.
It’s actually a very simple concept. It is basically an electromagnet attached to a permanent magnet — it’ll hold any metal object exactly as you’d expect — but if you run current through the inductor attached to it, the magnetic field created by the electricity will temporarily cancel out the field of the magnet — thus freeing your object being held. Since gravity is pretty fast acting, this impulse of current doesn’t need to be very long, only fractions of a second.
Now the real question is how big could you go? We covered another project a while ago called Open Grab which discusses the possibility of using technology like this in Quadcopters.
For a solution that uses no power at all take a look at switchable magnet clamps used for welding — they’re pretty cool — but patent protected of course.