It's not free, but X Plane is very good. It's got the realism of Flight Gear though, but the graphics are closer to MSFS. There are loads of great free add ons to it aswell. The only downside really is the scenary is not brilliant. North America has very good terrain, the rest of the world is more basic but still good (better IMHO than Flightgear anyway), but without having add ons the world is very empty, and whilst airports have accurate Runways, Taxiways and Aprons, they mostly lack buildings. There are some great scenary packages available though, many of them free (or cheap payware), but they are quite limited in numbers, and only tend to cover major airports. There are many planes available though, both freeware or cheap payware (e.g a couple of pounds for a plane with many livery varients). To be honest, that was what drew me to it over Flightgear, which is relativly limited in terms of planes (especially big commercial stuff), and many of them are very out of date, plus most only have one livery (X Plane seems to have a much bigger base of active add on developers, which is an advantage, plus the fact that is does allow payware add ons).
