Plasma Sky is a fast paced space shoot-em-up for iOS and Android featuring 80 levels, lots of power-ups, and multiple game modes. It's universal, and there are no in app purchases.
There's lots of kinds of enemy ships, and snakes, and your ship powers up and can get different kinds of ammo. There's a regular shield, but also a spike shield that lets you ram enemies. There are bosses! And there's lasers! And you can get a wingman who can power up too! And you and your wingman can get lasers! And there's other stuff! And it barely costs anything!
Source: The Game Website.