Shimano creates the Ultegra CS-HG800 Cassette to fit on a mountain bike hub, so those running big touring setups or cross-bikes can now run the smooth shifting feel of Ultegra, and with the huge 11-34 tooth range you won't have issues finding the sweet spot to pedal your way through any adventure.
Shimano's experience in machining and engineering allows for exceptional amounts of technology to go into tooth design, shaping, and shifting ramps. Made from nickel-plated steel, the cogs themselves withstand corrosion and are tough enough to hold up to abuse, which leads to a cassette that stays smooth shifting and precise for longer than before. By opting for nickel-plated steel, it does add a bit of additional weight, but allows for a price that's much friendlier to those of us who have to shovel out our own cash for components.
The CS-HG800 comes in only one size, but has a large enough gear range to work for most riders at 11-34-tooth. With a gear range this large it's important to pair it with the correct derailleur, as it will not work with a short cage, and requires medium or long cage derailleurs to shift properly.