Linux
Linux (or GNU/Linux) is a family of open source Unix-like compter operating systems based on the Linux. It comes in many “flavours” (see e.g., https://distrowatch.com/), offering unparalleled power/felxibility compared to commercial alternatives (e.g., Microsoft Windows).
Though the learning curve is somewhat steep, I find using Linux in my day-to-day life to be both efficient and pleasant.