The quite frankly wonderful Glitchskier is on sale for iPhone and iPad right now for just 99p / 99c. If you've not played this one yet, then it's basically a no-brainer at that price.
The game is set in a collapsing computer system and mixes retro awesomeness with super sharp modern shooter ideas. The end result is something spectacular and unique.
At review we called it "a thick and engaging shooter that does things its own way," and then gave it a shiny Silver Award.
It's brilliant, it's cheap, and you can click this link right here to go and grab it before it goes up in price. Don't thank me, it's all part of the service.