Prisma for iPhone
Prisma is an iPhone app that serves one purpose well: to transform your photos into works of art. It's only been around a few months but I've seen its results all over social media.
It doesn't just apply a basic filter or color palette; it "repaints" the image from scratch in one of 30 styles made to emulate a wide variety of art techniques, ranging from impressionism to abstraction to comic book/cartoon to stained glass to ancient Japanese woodblock prints...the list goes on.
I'll give you an example. My 4yo son recently went to a friend's rollerskate birthday party and I took this photo of his first skate:

Photo unedited except for resizing down to 1920px wide at 40% file quality. Blogging! ?
Image quality isn't fantastic, but in my defense, I was crouched down and rolling on my own skates while shooting. Anyway, here's that same photo run through a handful of Prisma's "filters" (which doesn't quite seem like the appropriate word but let's roll with it):
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"Curtain" (84%)

"Composition" (70%)

"Dreams" (80%)

"Coloured Sky" (100%)

"Speed" (69%)
Pretty neat, huh? Those percentages in parentheses are something you can adjust while previewing each filter; simply swipe left and right to adjust as desired.
What's most crazy to me is that Prisma is completely free on the iOS App Store, with no in-app purchases to speak of. If I were them, I'd have put 20 of the filters behind a "Pro" upgrade or something. I wonder if they'll release more filters at a price later down the line.
[Side note: Prisma's most recent update (v2.6, released October 6th, 2016) added the ability to apply effects to videos but I haven't yested it yet. This should be fun to play with.]