Stable Diffusion
A diffusion model was moved into a compressed latent space, which let it run on an ordinary graphics card, and the weights were released openly.
Why it matters
Image generation left the control of a few companies: anyone could run the model locally and change it.
Computing in latent rather than pixel space cut the cost by an order of magnitude. The consequences were social as much as technical: versions fine-tuned on particular artists' styles appeared at once, and the question of rights over training data moved into the courts. Open weights also meant any built-in restriction could be removed, an argument that continues.