If you've got an eye for quality, you've probably dropped some serious coin on the best toothbrushes money can buy - Oral-B's Pulsar toothbrushes. The bristles vibrate and scrub your teeth clean - but once the bristles begin to fray, it's a shame to throw away the whole thing. Turns out, the vibrating mechanism can be easily extracted and re-purposed into this lovely item. Make as many as you like!
To remove the core from the toothbrush, unscrew the handle and use a pair of pliers to gently squeeze the battery and metal contact (without damaging the battery) and pull straight out. Insert as-is into the button shaft.
Or, if you prefer the twist-on option, print out those two parts out (twistbase.stl and twistshaft.stl), extract the core from the toothbrush per the previous method. This time, however, bend out the outer contact a little so that the battery can't simultaneously touch both ends (just a millimeter gap will do). Insert with the button in the 'on' position and screw on the cap.
NOTE: the twistbase goes with the twistshaft - they're not compatible with any of the other three parts!
If you found this useful, please make sure you share a picture (it makes my day each time someone prints something I designed) and be sure to peruse my other submissions which are almost as varied as they are useful and many:
March 30th, 2017 update: Added skirted versions. They print more easily on average entry level build environments (like mine). :)