Or do you just toss them out?
I also use them for cleaning....in tight spaces, corners of window screens, narrow edges. They're great for the car kit and brushing off the battery before you go to fill it. Spatter painting is fun with them, too! I also use them to apply streaks of grey hair at Halloween.....I spray the grey onto the toothbrush and grab a hunk of hair and run the toothbrush down it.
I use 'em when I'm refinishing furniture, too......to get the sawdust out of tight spaces or to get the stain or finish into those same spaces.
I also use them to anchor skinny trunked plants. Brush side down holds them in an upright position and the plastic doesn't decay like wood skewers or bamboo.