A technique, published in 2015 by Leon Gatys and colleagues, that uses a neural network to redraw one image in the style of another. It was among the first AI art tools to reach the public.
I remember if you wanted to do style transfer, just getting your hand on the trained models was a puzzle, because that's one of the things you had to do.
Tom White, Tom White & Gene Kogan on the Birth of GAN Art
There's also been a lot of work on things like style transfer as an actual feature of the GAN rather than an internal implementation component.
Ian Goodfellow, Ian Goodfellow on Inventing GANs