What do you know? Think Geek has taken their excellent chrome suction joystick for the iPad, shrunk it down and brought it to the iPhone and iPod Touch. $17.99 will buy you one, and while I’ve always thought these miniature controllers were more trouble than they are worth, the iPad version of Joystik-It works pretty well.
If you do a lot of gaming on your iPhone, specifically in games which have a virtual D-Pad, you could do worse than one of these.