Can we use a computer fan with magnets attached to its blades, and another magnet, to generate electricity, as shown in this video?
Now that the link is to a YouTube video, I will take a brief crack at it.
The video is an illusion. There is no "free lunch" when it comes to energy generation - that is well established. If your magnet pulls the small magnets on the fan blades towards it, it will do so in both directions - there would be no net rotation. So let's hypothesize what the creator of the video might have done to create this trick:
I believe that the fan was rigged with a simple battery and a Reed relay. A Reed relay is a magnetic switch that closes when you move a strong magnet close to it. This battery could then power the fan, and the power could be put on the wires so it can light the bulb.