My family had an old not-so-good piano in the house that was given to us when I was still a baby. I learned to play on that thing, and that turned into a lifetime of being a musician. I play my guitars and keyboards at least an hour every day.
So, that's the first thing (and most important thing in my life probably) that I thought of, but if it has to be something that I bought myself as an adult, this is right up there as a recent purchase:
Shaves the head and trims any type of beard I feel like growing super well. $19.95.