If you're of the opinion that games these days are entirely too easy, what with automatic saves, regenerating health, and veritable labyrinths of handy chest-high walls, then you'll want to pick up side-scrolling classic Metal Slug. It harkens back to a simpler time (namely 1996) when men were men, women were women, and gigantic tanks bristling with missile launchers were waiting at the end of every level.
If you're unfamiliar with Metal Slug (and didn't pick up the somewhat more modern Metal Slug 3 when it came to the Play Store earlier this year) it's a side-scrolling shooter in the vein of Contra. Metal Slug's unlimited ammo is matched only by its unending supply of enemies, which attack en masse almost constantly. Every once in a while you'll come across vehicles for a boost to both your armor and firepower, and you'll want to hang onto it for the boss at the end of all six levels.
For all its machismo, Metal Slug is also one of the more light-hearted shooters of its time, with plenty of fun in the enemy animations. While it remains mostly unchanged in its Android debut, you've got the extra ability to select your stage. It's a reasonable $2, compared to $7 for Metal Slug 3.