Which organ can regrow after most of it is removed?

The liver. Remove up to about two thirds of it and the remainder grows back to full mass within weeks, which is what makes living-donor liver transplants possible - a piece is taken from a healthy person and both the donor's remnant and the transplanted section grow to the size each body needs.

It is regeneration in a loose sense rather than a strict one. The liver does not rebuild its original shape or its lost lobes; the remaining tissue enlarges and multiplies until total function is restored, guided by signals that appear to measure output rather than form. What is being regulated is capacity, not anatomy - which is why a transplanted lobe grows to fit its new owner rather than reverting to its old size.

It has to be able to do this because of what it does. The liver runs several hundred distinct processes - clotting factors, bile, cholesterol, protein synthesis, storing glycogen, breaking down everything absorbed from the gut including anything toxic - and it takes the chemical damage that comes with the job. The Prometheus myth, in which an eagle eats his liver daily and it grows back overnight, is either a remarkable guess or evidence that somebody in the ancient world had noticed.