Level Passwords:

2    72308
3    04070
4    82048

Weapon Select:
Earn 10 points after killing ten enemies, then pause the game. Now enter the following:

Up, Up          Laser          
Down, Down      Big Bullet      
Left, Left      Bouncing        
Right, Right    Rapid Fire      
B, B            Health Refill   
Select          No change       

Thanks to Revolution reader Adolfo Ramirez!