“P” at Nippleworks reveals that he uses safety glasses from the hardware store for cyclocross racing. Eye protection is helpful for commuting, too: I know a handful of people who’ve had to recover from a bug flying into their eyes. For riding at dusk or in cloudy conditions, amber lenses improve contrast. Clear glasses are best, of course, for riding at night.
My favorite cool weather cycling gloves are CLC Safety Tradesmen gloves that I probably bought at Home Depot or maybe Lowes a few years ago. They have the perfect padding for my hands, a great fit, and the all important snot wipe on the back of the thumb.
Before the advent of inexpensive, reasonable quality bike lights, I used to get most of my bike lighting and reflective bits from the auto parts store.
What other helpful cycling stuff can you buy from the hardware or home improvement store?