I studied diamagnetism in university as it is an interesting phenomenon not usually observed in day to day life.

Did you know that you can levitate a frog*?

Frogs are mostly made up of water, which is a diamagnetic material.  Diamagnetism is the opposite of ferromagnetism.  When you bring a magnet close to metal, there is an attractive force, and the metal "sticks" to the magnet.  In diamagnetism, there is a repulsive force.  This means that you can actually cause repulsion to "levitate" something against gravity.

In the photograph of the frog, there is a very strong magnetic field, and the frog is pushed up away from the magnet against gravity, which makes it essentially weightless.  

Other materials are also diamagnetic.  Bismuth metal is one such material, and in fact the most diamagnetic material.  Diamagnetism is still a very weak effect, and even the force from Bismuth is minute.  Another very diamagnetic material is highly ordered pyrolitic graphite (HOPG), which is essentially layers of graphene.  HOPG can be purchased very cheaply and it's fun to levitate using magnets.

Here's a thought: You can levitate HOPG on a permanent magnet and it will stay levitating basically forever.  Where does the energy come from to counter gravity?  (it's a trick question)

"Permission granted for this photo to be licensed under the GNU-type license by Lijnis Nelemans, High Field Magnet Laboratory, Radboud University Nijmegen."