I've just found a couple of more top-down shooters to add to the list (I'll add more posts later). Crimsonland is developed by 10tons, published by Reflexive and you can find a downloadable demo of the game here.
Now I wonder why they call this game crimsonland - could it be because it's the color which dominates the ground after playing the game for just a couple of seconds? If you like the game then you should try the two-player mode; it's excellent. A friend of mine and I played it with one 360 controller and a mouse/keyboard combo and both control schemes worked great.