Ikachan

Ikachan (いかちゃん, "Squid") is a freeware computer game created by Daisuke Amaya, art-name Pixel, and the predecessor to Cave Story. In the game, the player plays a squid named Ikachan, who swims through an underground cave, meeting and helping other creatures.

Gameplay
Ikachan features unique gameplay; as opposed to using the arrow keys to move, the player uses the left and right arrow keys to tilt Ikachan in the respective direction. The player must then use the 'z' key to swim in the direction Ikachan is pointing. Thus, Ikachan cannot swim directly to the left or right at first, but he later acquires an item that allows him to charge horizontally. Also, while being unable to hurt enemies from the start, once Ikachan receives an item called the Dunce Cap, he can attack enemies by ramming them with the point of the hat.

Source: Wikipedia, "Ikachan", available under the CC-BY-SA License.