The name says it all.

Here is a link to a list of great things to do with liquid nitrogen. My favorites are:

• Freeze a can of shaving cream and then peel the can away from the
  cream. Put the canless cream into someone’s car. Let the oven-like heat
  from the car’s sitting in the sun defrost the shaving cream. 2 cans
  will fill an entire car.
• Overclock your processor
• One of my favorite liquid nitrogen demonstrations is to place an
  egg (raw in shell)in a container and cover with liquid nitrogen. After
  the nitrogen has evaporated, take the egg out smash it with a hammer,
  it appears to be hard boiled. As it cools it returns to white and yolk.
• Get a couple liters in a container, and dump it on the floor in the
  direction you want the debris to travel. It picks up everything in it’s
  wave and if it hits a wall, the wave will boil off and deposit the junk
  there. Now all you have to do is go around the perimeter and sweep up
  the clutter.
• Take a ping-pong ball and poke a small hole in it. The hole
  has to be tangental to the sphere of the ball. When poking the hole use a
  pin and the pin should be almost flat agianst the ball. Basically you
  want a hole in the side of the ball that will cause the ball to spin.
  Submerge this ball into the liquid nitrogen and let it fill up. Place
  the ball on a table and watch it spin. As the nitrogen goes back to a
  gas it will rush out the hole and presto!! It’s pretty cool. If it does
  not spin try placing your hand on it to warm it to get it started.
• Sometime you may not have the time to cool the bottles in the
  fridge before serving. In this case, fill all the glasses with warm
  champagne, add a few ml of LN2 to each glass and serve immediately,
  while the LN2 is still steaming. This one is really cool.

1001 Things To Do with LN2