FlightGear is a realistic flight simulator, originally started 1996 as an indie project by a single person, but currently is progressing as an open-source project with many volunteers working on it. The simulation caters mostly to civilian flight, but later an add on supporting dogfights was released, and some bomb dropping scenarios are available with certain aircraft.
The downloads are split into several parts that are matched for each release: a base package, containing the game and a minimal area (San Francisco bay area) and few (15) aircraft; a large aircraft package for all the compatible aircraft (currently 400+); a large (~12GB) scenery package for flying outside the starting area, and the source code package for those who need to compile their own version. If you don't have the full scenery package locally, additional areas can be automatically downloaded as you fly over them.
There are server packages for setting up your own multiplayer server, if you don't want to fly on those already setup by others. The number of players depends mainly on available bandwidth, and it can be configured as an open or closed LAN server. In multiplayer is also possible to share a cockpit in a single plane between two pilots.