Toothbrushes have far more uses beyond brushing teeth. Toothbrushes have long been part of the professional cleaner's toolkit. So instead of throwing out your old brushes or storing them in a moldy cup, save one to make your house look shiny. 

Cleaning Tile & Grout

Tile is porous. It easily absorbs dirt, and stains can become near-permanent if not wiped away immediately. Fortunately, you can easily clean your tile if you have an old toothbrush lying around. Simply apply bleach or tile cleaner to the tile and then scrub vigorously. Ensure that your toothbrush has soft bristles. Hard toothbrush bristles can scratch and ruin your tile. 
Grout is more difficult to clean than tile. Dirt easily collects on its surface and mildew can begin to grow if there is enough moisture in the environment. To get rid of the grime and dirt, apply hydrogen peroxide and baking soda to the grout and then wait. After a couple minutes, begin to scrub vigorously until the grout is clean. 
Both tile and grout are susceptible to acidic soaps and cleaners. Checking to make sure that your cleaning agent is non-acidic before use will save you the pain of having to replace or repair tile and grout. 

Cleaning Animal Cages

If you own birds, rabbits, hamsters, or any other caged pet, you are probably used to scrubbing away droppings and food remnants. Sometimes droppings and food can resist the usual cleaning techniques (especially in the case of bird droppings). A toothbrush with stiff bristles will be able to scrape off just about anything stuck to your pet's cage.
Just apply soap and anything else you would typically use to clean your cage and then aggressively scrub at the areas your sponge was unable to fully clean. 
The unused toothbrush your dentist handed to you will probably suffice for this task. Just remember not to brush your teeth with it after you clean off your pet's cage. 

Cleaning Keyboards

Ever taken the keys off your keyboard? No? Well try it, and you will probably find enough dirt to fill a sandbox. All the crumbs from our late night Netflix binges end up under our keyboards, which is gross. Keyboard gunk is unlikely to affect your typing performance unless you have a lot of it, but it is unsanitary, and you should probably clean out your keyboard at least once. 
Carefully remove your keys (take a picture beforehand, so you remember how it goes back together) and take a moment to gaze at all the dirt you didn't know was on your keyboard. Then flip the keyboard over a trash can and use a toothbrush to scrape off the especially sticky or gooey parts that refuse to drop into the trash. 
Avoid water at all costs unless you have a waterproof keyboard (you probably don't), and do not use soap or any cleaning agent. But if you want to scrub with water, use a little bit of water and a paper towel to wipe down each key before you replace them. 

Cleaning Your Entire Bathroom

If anything in your bathroom is dirty, a toothbrush will probably be able to clean it. Most bathrooms have hard surfaces which are easy to scrub with a little soap, water, and a toothbrush. If you have mildew growing, just apply soap and scrub, but remember to check what exactly you are scrubbing. Tile, grout, and walls with paint might require different cleaning agents to prevent damage. 
And you bet your toilet can be cleaned with a toothbrush! Just apply soap and scrub, the toilet won't be damaged, unless it is one of those electrical toilets. 
Mold and mildew are usually a problem in bathrooms thanks to the steam made by our hot showers. It usually grows in cracks in the bathroom and in between tile, so you may have to poke around the corners of your bathroom to ensure that you are mold-free. If you do find mold or mildew, just scrub it away with a toothbrush and a little soap.
Tip: check the underside of faucets, shower heads, and the pipes below your sink for mildew and mold. These areas are often overlooked and usually, have the most mold.