Arrow keys to move, space to shoot. Press B to pause the game, and press V to toggle music on/off. You receive an extra life every 500 points. Your character will wrap around the edges of the screen, like in Asteroids.
I just re-submitted this after getting some complaints. One of them was that the screen was too small, so I made it bigger. I wasn't sure how much bigger to make it, so I just made it 100px in each direction. If that's not right, let me know. It's easy to fix.
Update: It should be a bit more zoomed out now, and bullets now travel much faster.