Earth Day is a nice little arcade-style game from eSoft Interactive based on the classic Space Invaders format. Pilot a small spaceship rotating around earth and fire away at alien craft, meteors and whatever else gets close. You can shoot smart bombs, lasers and even add a second spaceship to help you in your fight.
There are free life-time updates, great sound effects and music, high score ranking, and never ending levels. Hold on, doesn't that mean earth is doomed?!
Initially a bit slow to get going, once the action warms up and more aliens invade, Earth Day becomes a more exciting challenge.