— Watch it!
My first game was a simple two-player gravity simulation, where each player was given a multitouch trackpad and could create up to eleven gravity wells by placing their fingers on the trackpad. Gravity affects circular bubbles, which merge if they touch, growing if they're of the same color, and annihilating each other if they aren't. Players attempt to fill their target (the square areas in the middle of the board), and attempt to prevent their opponent from filling their target by annihilating their bubbles.
The game is written in Objective-C, making use of Apple's multitouch APIs for input and CoreGraphics for drawing.