Now go outside and look at the sky.
Cool Open Source: FlightGear
This is very cool: I've just done a lazy flight around San Francisco Bay with FlightGear - an open source project that has been going on for about fifteen years now. Not sure how I managed to not hear of this before, since I've been having bad withdrawal symptoms since I'd ditched Windows and Flight Simulator a few years ago.
FlightGear is very far advanced at this point and much about the architecture of the platform is very attractive. All the scenery files, plane models and the AI traffic definitions are done in XML - there is a lot of stuff to poke around in... not that I really need another time sink at this point...
Flying in FlightGear was very nice. Similar to Microsoft's FlightSim, but maybe a little bit more of a realistic aircraft handing (which is not always good for the casual pilot!). The scenery around the Bay was gorgeous and the clouds and sky (which is what a pilot will see the most) are near perfect, at least as good as FS9.
With versions for Mac and PC and as a free download, what is there not to like?