Remembered the game -- Crimsonland

[Safeway]

You control a guy with the arrow keys, and the crosshair with the mouse. It is a top down shooter -- you kill waves of spawning aliens, as well as the occasional boss. All this takes place in a large square on the screen. I played the game about two years ago, and was linked from ATOT. You could play the game in a browser, or download an executable.

You can get shield/life power-ups, and new weapons. Weapons include shotgun, automatic shotgun, flamethrower, machine gun, rocket launcher, and possibly others. There is also a nuke power-up that clears the screen. Each time you shoot, the path of all your bullets displays on the screen and fades out. It is a very cool effect with the shotgun, since it traces so many projectiles.

The waves of aliens become impossible eventually, but I remember that a cheap code exists that makes you invincible.

Note: It is not Geometry Wars.

What game is this?

Edit: Crimsonland!

 

[Safeway]

Need help. This is bugging me.

 

[Safeway]

One more bump for the morning crew ...

 

[YOyoYOhowsDAjello]

I know this isn't it, but it has some of the same qualities you're looking for, so you can play it until you get the real answer
Zombie Horde 3

EDIT: Oh, I saw in the other thread that you were thinking of something similar to geometry wars

 

[Safeway]

You can also get power-ups that freeze the enemies, or make you shoot faster or run faster.

 

[Safeway]

It was Crimsonland!