'A tractor'.. (or 'The Universal', either way)... is an online game for PC. Play games, explore the galaxy, or make your own world with your own game design. Various game types are available - including complex social economies - and you can play it all for free. If you're the creative type you'll find extensive in-game tools and a powerful scripting engine to let you create new game ideas. Or you can just mess around and blow each other into ostriches. It's all cool, whatever.
This video deomnstrates the new 'cloud-hosting' system within The Universal, which makes creating your own world as easy as clicking a few buttons. As a world owner you have access to a range of gameplay and graphical settings, editors and tools, allowing you to change vehicle setups, landscape graphics, gameplay rules, audio, etc etc.