How can there be a potential difference if a magnetc field can't do work? If I move a wire through a magnetic field, then it should make a potential difference.  If I put it on tracks so that it moves in a straight line, then it should be able to continually move without any force acting on it.  However, it is still moving, so it should produce a potential difference, which I can use to power something like a light bulb.  Where does this energy come from?  
 A: When you move a wire through a magnetic field an emf is induced.
If the wire is on tracks and there is no complete conducting circuit and so no induced current then the wire will continue to move at constant velocity.
If there is a complete conducting circuit then there will be an induced current which,  according to Lenz, will be in such a direction as to oppose the motion producing it.
The current carrying wire moving in the magnetic force on it which is in the opposite direction to its movement.  
If the wire has no other forces acting on it then that force due to the induced current will slow the wire down.
The net effect is that the decrease in kinetic energy of the wire is equal to the electrical energy generated by the wire as it moves through the magnetic field which ultimately finishes up as heat due to ohmic heating (and light if there is a light bulb ) in the conducting circuit.  
If you want the velocity of the wire to stay constant then you must apply a force on the wire to oppose the force due to the induced current and so you must do work to maintain the constant velocity motion of the wire.


Using the symbols defined in the diagram is the resistance of the circuit is $R$ then the electrical power produced is $\dfrac{(Blv)^2}{R}$ whilst the mechanical power needed to keep the wire moving at constant velocity is $Fv = BIl\,v$.
As $I = \dfrac{Blv}{R}$ the mechanical power input is $\dfrac{(Blv)^2}{R}$ which is exactly equal to the electrical power output.
So it is not the magnetic field doing the work, it is you who is doing the work to generate the electrical energy.
A: 
it should be able to continually move without any force acting on it.

No the wire actually slows down due to the magnetic Lorentz force on the charges inside of it.  Its lost kinetic energy exactly equals the energy powering the light bulb, and energy is conserved.
A: The energy that can come to the wire from the magnetic field is proportional to the area enclosed by the wire through which magnetic flux penetrates. As long as that area is constant (zero for a simple line which does not enclose any area), moving the line around in the magnetic field does not induce energy transfer. So your assumption is wrong: The wire does not get energy from magnetic field in potential energy form simply because it is in the field.
When the flux through the are enclosed by the line changes, then the potential energy of the wire changes, in so-called electromotive force format which can induce work on the electrons by turning them into a current.
This phenomena is called Faraday's induction law.
