Commander. Not the most inspiring title in the world, to be sure, but there's a charming little game hiding behind it. A tower defence game without any towers, Commander plops you in the seat of a ship, zipping about from planet to planet and dropping various sorts of turret on the surface, ready to blast any incoming threat. Those turrets must be enormous and probably bring down property prices in the area, stretching out into space as they do, but they get the job done. The game's still in alpha but there's a demo available on Desura, as well as an option to pay £2.99 for this and future releases. See video below.
Blocky aliens and jolly little planets! Eye-embracing particle effects! There's nothing gritty or grim about this end of the world, it's like a firework display and a child's memory of a planetarium having a fun day out at a fairground. It plays well too and I like the way the enemy paths loosely conform to the gravitational pull of the celestial bodies around them, which allows for clever use of a gravitational turret to slow the blighters down.
A level editor rounds out the package, allowing the construction of sizable and complex gauntlets. Do try the demo if you are so inclined.