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 two main engines on Zvezda can be used to raise the station's altitude. This was done on April 25, 2007. This was the first time the engines had been fired since Zvezda arrived in 2000.
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.
In addition to the previous answers, in its most recent mission (May 2018) the Cygnus resupply spacecraft tested out its ability to give the ISS a boost.