A flight simulator for Windows and Mac, with Linux version coming up.
X-Plane is a realistic flight simulator that tries to be as completely tweakable as possible.
You can design any kind of plane or aircraft you want - real or imaginary - and it will calculate a credible flight model for that; You can also design and use any kind of scenery you want. The game, of course, comes with all sorts of planes and has scenery from all sort of interesting places.
It also models all laws of physics accurately; In fact, not only you can fly on Earth, but in Mars, too - the game includes all required Martian parameters and terrain. (Flying there with terran planes is pretty damn tricky though...) The new versions apparently also support very frosty features found in the competiting products, like virtual ATC networks.
X-Plane is also a rare product in that it's the first FAA-approved training simulator that's available on ordinary consumer market (though you also need a cockpit simulator and a flight instructor to make it count...)
While the game itself is interesting on its own right, what's even more interesting is that it's made by a "guerilla developers" (that is, it's not being released by a big software house). Just one developer (Austin Meyer), in fact. The game is only being sold over the Internet for relatively modest prices. However, some software chains seem to carry it as well - I noticed that one major software importer - Sanura - in Finland distributes it! Cultural Act of the Year!
The only bad things about X-Plane are the fact that finding patches for old version is tricky (where's the 5.x patches so I can get to the latest rev???), Austin's web design skillz make my eyes bleed (Death, thy name is text-align: center;; Sorrow, thy name is A Whole Lot Of Capital Letters), and Austin has previously got to some small clashes with people (however, this is fortunately not quite of Dereksmartian proportions yet...) ... but I think the good stuff outweighs the bad by zillions of miles.
The game's website: http://www.x-plane.com/