I've been meaning to do something like this for a long time, and I'm sure a lot of these y'all already know, BUT when I found out about them I had never heard of them being used that way, and it saves money in the long run rather than than buying specialty equine products:
Ivory Dish Soap - great shampoo, even for my white pony. Just use it like you would any other shampoo, and get the original kind.
Desitin cream - I first found out about this stuff when Greta had some dry, flaky skin on her nose. Resident vet tech told me about this stuff, and it worked. It also works to help heal wounds and prevent scarring. Apparently it can also work on areas affected by rain rot or scratches after the crusts have been gently removed, washed with a providine-iodine scrub, and thoroughly dried. It's active ingredient is zinc oxide, so it can be used for all kinds of things, not just diaper rashes!
Listerine - a great mild antiseptic. It can also be used to prevent tail rubbing, with or without equal parts baby oil. Use the original amber-colored kind.
Petroleum jelly - rub it into thick chestnuts for 4 to 5 days, and they will peel right off and be show-ready.
Baling twine - is amazing. I'm sure we all know of its many uses, but did you know it also works well as a sweat-scraper? It works especially well for that in places like down the leg.
Got any interesting stuff? I would kind of like to find a much cheaper liniment alternative to Absorbine's liniment and Show Sheen, as $20 a bottle can get a bit expensive haha.