The International Space Station is constantly losing orbital energy due to atmospheric drag. How does station-keeping work for the ISS? What sort of propulsion system is used to keep it in the desired orbit, and where are these thrusters located? I imagine it could be as simple as pressurized nitrogen.
More recently, ESA's ATVs have also been doing reboosts using MMH/N2O4 thrusters.
On the order of 2 m/s per month is required.
In addition to Progress and ATV vehicles, docked at the aft end of Zvezda, using their main engines to boost the orbit, the Zvezda module also has engines that can be used. In fact, the Progress cargo vehicles can refuel the Zvezda module.
The US plans to send a VASIMR engine by Ad Astra, to the station to test its ability to provide thrust to the station for orbital maneuvers.