China based game accessory maker Firstsing offers a mini USB Cooling Fan for the Nintendo Wii console.

From my experience with the Wii, you do not need a cooling fan. But maybe if you are living in a hot region without air-condition then you might want to add some extra cooling.

The Wii mini fan plugs into the USB port of the Wii. The fan has one USB port built-in. As far as I can see from the product photo the fan covers the 2nd USB port of the Wii when plugged in.
The company sells to OEM or dealers and not direct to consumers. I assume this Wii accessory would only cost a couple of $.
Via Firstsing site. More Wii News.