These have been around for a long time but are still a fantastic way to convert a normal bike into a single speeder. The rear axle is concentric meaning you can tension the chain by rotating the rear wheel around its axle.
White Industries make them for disc and non-disc and you can bung them on your mountain bike so they must be pretty tough.
If you’re wondering how you mount a disc brake with a concentric hub then see below, it’s a concentric disc mount:
The hubs are flip/flop for fixie or single speed, you choose.
White Industries also make some other desirable single speed goodies.