According to this article , you can use toothbrushes to clean bike chains .
I have to admit I never did this myself; I have used dish brushes for similar purposes.
lifehacks.stackexchange.com/users/5345/Glorfindel
I use old toothbrushes for cleaning multiple surfaces in multiple ways.
For grout: I drill a small hole in the center of the head, push a small bolt through, add nut, tighten, cut off the toothbrush handle and install it as a bit into my drill.
Instant power scrubber.
You can do the same with a larger scrub brush .
For cleaning small parts or jewelry: Apply appropriate cleaner to toothbrush and scrub away.
Much easier cleaning tool than a sponge or scrub pad.
lifehacks.stackexchange.com/users/19151/M.Mat
You can use a toothbrush and elbow grease to clean all types of surfaces.
Here are some other uses: Toothbrushes are better at cleaning inside glasses (for drinking from) than the average dish brush because the dish brushes are too large to fit inside small glasses.
You can use toothbrushes to brush caked-on dirt and mud off of your shoes.
Untested uses:
You can try using a toothbrush to remove pilling from sweaters.
You can try using it to apply facepaint or makeup at your own risk.
lifehacks.stackexchange.com/users/25488/John Locke
The uses of toothbrush is as follow: To dye hair near forehead without smudging.
To clean stainless steal taps.
To remove dust from wooden furniture specialy from that areas where we can't reach or the design of furniture does not allow us to clean directly.
To remove dead skin cells from your lips.
For painting.
lifehacks.stackexchange.com/users/26015/Chawlish Boy
I use worn-out toothbrushes for several purposes: to remove dirt under my fingernails, to clean files and sandpaper.
lifehacks.stackexchange.com/users/5307/Hobbes
You can also use a toothbrush to clean out clogged shower drains.
If the drains tend to get clogged with hair, it's convenient to use a toothbrush to clear it out (rather than using your hands/fingers directly).
lifehacks.stackexchange.com/users/25995/sq99
There are many possible uses for old toothbrushes: Cleaning the Bathroom Polishing Shoes Cleaning the mold in a Washer Cleaning the bottoms of shoes Cleaning ovens
lifehacks.stackexchange.com/users/26033/andrew_reiprich0519
Your question mentions "spare" toothbrushes, so I'll add this suggestion.
Toothbrush sanitizers are becoming increasingly popular, especially when in close proximity to a toilet (Google for details).
Rather than, buy another gizmo, I have 7 toothbrushes, one for each day of the week.
At end of the week I put them through the dishwasher.
Toothbrush sanitation solved!
lifehacks.stackexchange.com/users/25438/Fuligo septica
Is there any tips to reuse the wasted tooth brushes in a useful and interesting way?
How to use the spare and wasted tooth brushes?
lifehacks.stackexchange.com
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lifehacks.stackexchange.com/questions/19428
lifehacks.stackexchange.com/users/25649/ASIM
Old toothbrushes can be used for a wide variety of cleaning tasks. They can be used to clean surfaces, jewellery, drinking glasses, shoes, stainless steel taps and even finger nails. Aside from cleaning, they can also be used for painting or face painting. The options include using to clean surfaces, clean inside drink glasses, remove dirt from shoes, pile from sweaters, apply makeup, to dye hair near forehead, clean stainless steel taps, remove dust from inaccessible corners, remove dead skin from lips or dirt from under fingernails, clear hair from clogged drains, clean the bathroom, polish shoes, clean away mold in a washer, clean shoe soles and clean ovens and for painting.
Old toothbrushes can be used for a wide variety of cleaning tasks. They can be used to clean surfaces, jewellery, drinking glasses, shoes, stainless steel taps and even finger nails. Aside from cleaning, they can also be used for painting or face painting. Old toothbrushes can be used to clean various surfaces, bike chains, shoes etc. they can also be used to remove pilling from sweaters and to dye hair near the forehead without smudging. Additional uses include as a nail brush, to clean sandpaper and clean clogged shower drains.
Old toothbrushes can be used to clean hard to reach or dusty surfaces in the bathroom or on furniture, to clean clogged drains, to remove mud from shoes, to clean inside drinking glasses or to clean mold from washers. They can also be used for removing dead skin, removing pilling from sweaters, painting, face-painting, or to apply hair dye. Toothbrushes can be used for a range of cleaning tasks, whether cleaning surfaces, drains or shoes. They can also be used for cosmetic applications and even painting.
Use for cleaning various surfaces - attaching to a power drill as needed - or inside drink glasses.  Use to remove dirt from shoes and piling from sweater and to apply makeup. Use to dye hair near forehead, clean stainless steel taps, dust from inaccessible corners, to remove dead skin from lips or for painting. Use to remove dirt under fingernails and to clean files and sandpaper. Use to clean out hair from clogged shower drains. Use to clean the bathroom, polish shoes, clean away mold in a washer, clean shoe soles and clean ovens.
Old toothbrushes can be used to clean various surfaces, bike chains, shoes etc. they can also be used to remove pilling from sweaters and to dye hair near the forehead without smudging. Additional uses include as a nail brush, to clean sandpaper and clean clogged shower drains.
Toothbrushes can be used for a wide variety of cleaning tasks to great effect. Toothbrushes can make good tools for a range of cosmetic tasks, like facepaint or makeup. Toothbrushes make a great tool for cleaning clogged drains. Toothbrushes can be used to remove piling from sweaters. Toothbrushes can be used for painting.