The supercomputer in The Hitchhiker's Guide to the Galaxy that calculates the answer to life, the universe, and everything is Deep Thought.
Deep Thought is built by a pan-dimensional, hyper-intelligent species whose three-dimensional manifestations are ordinary white mice. The computer is described as being roughly the size of a small city and is designed to solve the ultimate philosophical question.
After seven and a half million years of calculation, Deep Thought produces the answer “42.” The joke is that nobody actually knows what the corresponding question is. Deep Thought therefore designs Earth, an even larger computer intended to discover the question, but Vogons destroy Earth just before its computation finishes.
A common mix-up is confusing Deep Thought with Earth, which is the later computer, or with Heart of Gold, Zaphod Beeblebrox’s starship. Milliways is the Restaurant at the End of the Universe, not a computer.