The most common geared hub on the market is still the Sturmey Archer. These come in multiple speeds. All of these hubs are serviceable but parts for some of the models are difficult to come by. After servicing it is essential to replace the cables. More often than not with these hubs it is the cables that are at fault before the hub.
Most modern hubs are not serviceable. The Shimano hub can be cleaned and re-lubricated and the cone bearings replaced. However if the fault is greater than this then a new unit can be slotted into the old casing. This saves times rebuilding the wheel. The Rohloff units although expensive can be returned to Rohloff for full servicing. It is rare that these units fail. They do periodically require having their gear oil replaced, and the cables again are an essential combination for both the Shimano and Rohloff units to work well.