Tub Mats
I guess I'll kick this off with something top-of-mind.
I spent nearly the last decade in a house with people who don't believe in tub mats, using the most slippery shower I've ever experienced in my life. The only thing that kept me from falling on my ass most days was the little towel bar that was never meant to support the full weight of a human being.
Enter the brand new tub mat that I got for the not-brand new tub in my even less brand new apartment. The simple act of being able to take a shower without fearing death has been the single greatest improvement to my life since I moved out on my own.
Thank you, humble tub mat. You're doing the work of the gods.