Space Strafer is a single button game of spaceships, lasers, asteroids and valuable crystals. Intricate scoring mechanisms make it possible to layer risk multipliers and chains to achieve huge bonuses, while a variety of ships cater to different play styles. It's a free game with no IAP's, there is a single ad at the top of the death screen which doesn't force you to wait around, so it's fairly discreet.
Looking to buy the "Beta", which can be purchased with crystals collected while strafing. (Mobile - Android)