Flightgear

[ShadowRunner]

FG is a simulator available for mac, windows and linux.
It's open source, so free to hack and mod etc.
It's free to download as well.
It has multiplayer and several servers. The UK server has around 20-30 pilots during late evenings.
Pilots can take over Air Traffic Control, which makes it fun. You'll get shouted at if you connect then start engines on the runway.
It comes with various models, including Boeing 777, but others like A380 and 747 and B52 etc are downloadable.
Scenery is handled by "terrasync" which downloads in advance the scenery you're heading to.
Scenery is available for the whole world, except empty ocean expanses and some of Antarctica.
For free software it's impressive. For a trainee pilot, it's also very useful, with proper Nav/Radio functions.

The only real negative is that you need to configure it properly before flying. (I started with bad fps and was crashing a lot, no joystick either)
It is not advertised as a game, in fact it has been used in colleges and by NASA, but it has good reviews as a game also.
http://en.wikipedia.org/wiki/FlightGear
Pro Flight Simulator and Flight Sim Pro etc are basically Flightgear being repackaged and sold. So don't pay, download FG.

DOWNLOAD: http://www.flightgear.org/download/main-program/

The multiplayer game seems to only host San Franciso Oakland Airport vicinity. Select runway 28L for for easy take-off.

[chin.democ.]

I've bookmarked your links, thanks. I've played Microsoft flight sim 2002, and some of their earlier stuff including a WW2 version. I once tried to land a 747 at London city with disastrous consequences as you can imagine!

[ShadowRunner]

Very brave, I have trouble even finding it...

It's has no WW2 mod or combat sim developed for it I think unfortunately. Gotta play Aces High or IL-2 etc.