A $15 coffee maker. It's such a cliche, but making coffee at home (or, more often, at the office, where I don't directly pay for the fancier machine or the nicer coffee!) saves me ridiculous amounts of money, makes me happier to wake up, and generally improves my life.
Second place, though, I'd have to give to an indulgence--I bought my first guitar in November, for $300. I anticipate decades of enjoyment with it, but I'm not going to pretend it was a very Mustachian purchase, especially as I was too nervous of getting cheated (I know basically nothing about what makes a guitar good or bad*) to buy anything but new.
*I also plan to try to avoid learning this, which I know sounds counter-intuitive, but I've found that there's a danger for amateurs in learning how to identify the best. Amateurs rarely need the best, and usually don't need to upgrade--at least, I certainly don't. I didn't need a super-fancy Nikon to take shots of my friends in university, and I sincerely doubt I'll need an incredible guitar to haltingly strum out One Direction songs. What I *will* need is to avoid the temptation to find flaws in the lovely workhorse guitar I already own!