Play the free online flash game P.O.D. on Kongregate. Here's the description:
This is my re-interpretation of the arcade classics Space Invaders and Missile Command, with some new gameplay elements.
P.O.D. is an arcade-style game of all-out alien bombardment. Your city is under attack by aliens, and they don't give up easily! Take control of different units and power-ups to blast those suckers out of orbit. You are Planetary Orbital Defense.