This is version 1 of my BB-8 droid from the new Star Wars movies. It had pretty much no budget and acted as a great learning ground before I spent more money. A video of it in action is below.
For more about this droid, along with general design details, see my main BB-8 droid page.
Here's a video showing mine in action, as well as talking about some of the issues in getting one to work.
How the BB-8 droid (v1) was made
Starting from a toy remote control truck I made the complex "truck" shown below, which I refer to as the internals.
The brass tube extending up from the truck goes to a plastic disk that contains four magnets (see photos below.) Those magnets attract four other magnets attached to the bottom of a plastic cross. That plastic cross is what goes on the outside of the ball and acts as the base for any dome or head, such as the cardboard one shown at the top of the page above.
The magnets were purchased from a hardware store (Home Hardware) and the packaging is shown below. They are neodymium magnets with dimensions 0.47 inches (12mm) in diameter and .11 inches (3mm) thick.
Due to the weight of the plate at the top of the brass tube and the things mounted on it, the truck would sometimes tip over inside and the wheels would lose traction (see photos below). The solution to that was to add the side supports. At the end of each side support was a roller with a ball (a marble) in it that could move in any direction along the inner surface of the globe.
The roller was made by using a pipe cutter to cut one end of a brass tube. That left a burr which kept the ball, a marble, from coming out. A piece of wooden dowel was then inserted into the tube to prevent the ball from coming out the other end and for the ball to roll against.
The result was that the truck would no longer tip over.
Video - How to Make BB-8 Droid Build (v1) from Star Wars
Here's a video showing the making of this BB-8 droid version 1 going through it step-by-step, including a lot of the experimenting and reasoning involved.