Amp modeling
Digital emulation of a guitar amplifier and its speaker cabinet, so the tube amp can stay at home.
Line 6 led the way with the AxSys 212 in 1996 and broke through two years later with the POD, a red kidney-shaped box that let guitarists record with headphones on and neighbours intact. Early models sounded like what they were. Fractal Audio's Axe-Fx brought the technique onto touring stages from 2006, and Christoph Kemper's Profiling Amplifier changed the method in 2011: instead of modelling the circuit, it measured a real amp with test signals and imitated the result.
The job usually splits in two. The amp is non-linear and has to be simulated; the cabinet is effectively linear and gets captured as an impulse response. In recent years neural networks have taken over the non-linear half, with Steven Atkinson's open-source Neural Amp Modeler as the most widespread example.