In cities, motorcycle taxis are ubiquitous. For quick trips across town, just stand on a corner for a moment and someone will offer you a lift - for a small, usually standard, fee of US$1 or less. Unlike their Thai counterparts, they do not wear any identifiable vests but usually will be wearing their helmet or baseball cap and standing next to their motorcycle.
Motorcycle rentals are available in many towns, with the notable exception of Siem Reap. Be careful if driving or riding yourself: driving practices are vastly different from developed countries. Local road 'rules' will also differ from city to city.
There are a number of motorcycle touring companies in Cambodia, such as Ride Cambodia Motorcycle Tours, that run single or multi-day trips across the whole country. This is great for those that want to get far off the beaten path and see the places that a tourist bus could never reach.