Blastz io is a great example of true io game with some simple graphics, but still exciting gameplay!
How to Play
You begin as a level 1 spaceship and you can:
collect color dots for XP
destroy enemies for XP
pick up aid kit for restoration of HP
As you reach a certain level of XP you'll level up and your spaceship will be upgraded too. Control of a ship is a bit tricky, it has a sort of inertia, so you've to get used to it. Your solely goal in Blastz io is to fight-fight-fight and get to the top of the leader board. It's not easy to get to the top, but very easy to lose everything you've achieved in a second.