I'm always looking for stuff that could be useful in the hobby:
discarded electronic parts- great for cylinders and engine parts
discarded electrical cords and electronic wiring- useful for armor tow cables, engine and aircraft wiring and plumbing and even aircraft seat belt buckles.
paper coffee filters- great for polishing
wood coffee stirring sticks- my go to tool for stirring and mixing paints
ladies emery boards- from the Dollar Store or Wally World, in multiple grits and some are padded.
discarded ceramic tile- used as the support cutting surface for pe.
discarded plastic tops from water bottles- used as disposable mixing palettes for paints and washes. Turn them over and use the other side to place a drop of any type glue for a precise gluing application.
left over kit's parts tree numbers- can be carefully shaven off and used as casting numbers on armor.
wood popsicle sticks- great for making wood fences, wood slats, and just about any king of diorama setting.
discarded plastic pill bottles- great for holding small parts and there are types with a double screwable top that will allow a figure to be temporarily secured to it, making a great handle to use while painting, and allowing the figure to be in the bottle until it's completed.
plastic food trays- I save some of the small Jello plastic cups to be used as disposable cups for decanting spray paints, holding small parts or mixing diorama terrain materials. The larger microwaveable disposable food trays can be used as temporary holding bins for a project's parts such as holding all those wheels for German armor subjects.
Cheap hair spray from the Dollar Store- great for the 'hairspray weathering technique'.
metal strainers from the Dollar Store- used for armor and aircraft engine and vent screens.
dried flowers and moss from the craft store- useful for diorama settings
wine bottle foil- great for making straps, belts, tarps, fenders and a ton of other things.
beer, soda, tea cans- for manufacturing parts such as certain aircraft seats, Schürzen for German armor, etc.
Kitty litter- best material I have found to use as rubble, sandy, or rocky terrain. Crush the material as fine as you like and is easily colored with craft acrylics mixed with water and white glue.
dried roots from the backyard- these make excellent trees. trim to size, spray hairspray on it and sprinkle ground foam to build up the foliage.
spices from the kitchen- spices like dried oregano, parsley or any leafy type will make great ground leaf debris.
sewing needle- I use one in my hand held pin vise with the eyelet slit open as a super glue applicator. The eyelet will hold the right amount of thin ca than can be precisely applied to the most difficult part to reach.
window cleaner from The Dollar Store- Huge bottle for a buck and works the same as Windex at a fraction of the cost. Great for stripping off acrylic paints from plastic parts, cleaning the ab, and it even works at cleaning glass!