It's the Captain Low-Rez game, Blastopod!

Author Comments:

Programmed by Tom Fulp, this game is like Atari on acid!
A KEY = SHOOT
S KEY = MEGA CANNON (must try this!)
DOWN KEY = SHIELDS
LEFT & RIGHT = MOVE SHIP
Your shield absorbs enemy life to raise your own health. The more you charge your shield, the bigger the impact!
Also, your Mega Cannon is enhanced when you have a fully charged shield!
The Captain Low-Rez series originally appeared on Newgrounds in January of 2001, but was never submitted via the Portal. It is in the process of being moved into the Portal system, for consistency.