No, I'm not talking about those exercise machines, diet supplements, "get rich in real estate" schemes, gasoline additives, genitalia enlargement pills, and other questionable products and schemes that are advertised on late night television. I'm talking about things that made by reputable companies, that are marketed and sold as legitimate products, that just don't work. My short list:
Grass patch - the combination of grass seed and pulverized recycled paper that you spread over a bare spot in a lawn to get it to grow back. Dig up the old turf to the point where there's nothing but exposed bare soil, chop it up a bit, spread the patch, water regularly, and ... nothing. Never, ever worked for me. I have much better results with grass seed that's raked or ground into the soil.
Lawn weed killers - again, a product that has never worked for me. The general herbicides like Roundup and Triple Shot are great for killing anything with roots, but the weed-and-feed products just seem to feed. Despite my textbook lawn care from the start of spring, and using them at their suggested intervals and application rates, crabgrass, chickweed and broadleaf weeds have taken over about a third of my front lawn. I'm digging it up and starting from scratch; sometimes, you have to destroy the village to save it.
Over the counter allergy medicine - nope, doesn't work for me. Claritin only works in heavy doses; enough to make my throat very dry. Benadryl puts me to sleep, and only makes minor improvements to my stuffy, dripping nose. Everything else has the power of a placebo. I use the good stuff -- Zyrtec, which requires a prescription. It works; even better than allergy shots have ever worked.