The German studio Egosoft built the X series on a single idea: create a universe that keeps running whether or not the player does anything in it. X3: Reunion, released in 2005, was the largest realisation of that ambition at the time and is still cited as a reference point for open-economy space simulation.

The surface layer is a cockpit view and flight between sectors, but the substance lies elsewhere. A live trading system runs underneath, with factories buying raw materials, producing goods and dispatching ships along routes. Prices move with supply and demand, so you can make your money through trade, through piracy, or by gradually assembling your own industry and fleet, at which point you are managing more than piloting.

That is also the game's main barrier. X3 has a famously steep learning curve, and the first hours are largely spent reading the interface and working out how the systems connect. Get past that investment and you have a game that supports hundreds of hours, because you set your own objectives.

By 2005 PC hardware could carry a project like this, with a large world and a lot of background simulation running at once. Egosoft also leaned on a long-lived modding community, which extended the series' shelf life considerably.

OldGame.cz hosts a profile with release information. If you want a space game where the real activity is building rather than mission-running, this one belongs on the shortlist.