WARNING: This is not a safe way to control a motor. Each I/O pin can only handle 40 mA of current. I would recommend using a H-Bridge or something along those lines.
When I say simple, I don't mean use a speed control. But control the motor directly without any external circuitry. Here's how:
How this came about:
I recently was working on an instructable about charlieplexing with an arduino. And I was wondering if the same principle would work with motors to some extent. So I came up with the idea that if you used a motor instead of an led you could have 2-way control of it & if you used 2 PWM (Pulse Width Modulation) ports you could have 2-way variable speed control for a motor with no external hardware!! So I decided to post my findings. Have fun! If you have any questions Please ask them.
I am not responsible for any thing bad that happens to you or to your arduino!