Quantum simulator
An ordinary program computing what a quantum computer would have done — indispensable for development, and hopeless beyond about fifty qubits.
Representing n qubits exactly requires 2ⁿ complex numbers. At 30 qubits that is a few gigabytes, at 40 terabytes, at 50 petabytes. The wall is steep and absolute, and it is exactly what makes quantum computers interesting — if they could be simulated efficiently, they would not be needed.
Simulators are still what you work against day to day. They are exact, steppable, and can show the full state vector — something no real machine can. You can also switch on realistic noise models to see how a circuit would fare on hardware. Clever methods such as tensor networks push the limit upwards for circuits with limited entanglement.