Here's one I rigged up. I took an 120mm case fan and old Tracphone Motorola cell charger and connected them. The fan is 12v DC and the charger is 5v DC output So I don't have to worry about the fan getting too much juice and burning itself out. It's only two wires, so I believe it will spin the fan one way or the other depending on which way they are hooked up or not work at all. Another way to run a PC fan is to connect it to a an old working PC power supply. To turn on the power supply without it being connected to a motherboard take a paperclip and tap the green wire outlet to any black one to turn it on;
VIDEO CLIP! (Be very careful doing this and don't shock the be-jesus out of yourself.) Here's some pics of my rig job, it had fancy blue leds which would have just eaten up some of that 5V so I snipped all the wires to the leds. =)