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There are, broadly, 2 categories for USB toys, useful toys and fun toys. Useful toys are gadgets which aim to fulfill a useful purpose but are of limited practicality due to the restrictions on the power supply through the USB port. Examples of these would be paper shredders and reading lamps. The fun toys have no practical use other than pure fun. Examples include USB missile launcher, USB boxer and USB whack it. These are especially popular in office environments as they provide an interesting distraction and can often relieve stress in a similar way to the classic 'stress ball'.
Power supply 
The USB specification provides a 5V (volts) supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25V and no less than 4.35V between the +ve and -ve bus power lines. The USB connector was originally designed as a replacement for the serial and parallel ports on computers. It was hoped that eventually a number of devices would be able to daisy chain together using the USB ports. The use of a USB port to power toys was probably not what they were intended for.
See also 
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