XBOX 360 ONLY, BRO.
Forza Motorsport is a racing simulator, similar to Gran Turismo. However, Forza is famous for its absurd amounts of customization and great variety of high detailed cars (they all have full working interiors, etc). FM4 just came out on October 11th, and I've been playing it constantly; it's damn near perfect.
The game has something like 500 cars (though probably more like 380 or 400 unique cars when you cross out the racing versions of the cars and their liveries) from all classes and manufacturers (HUMMERs, Prii, SSC Ultimate Aeros, Ferrari F40s, etc), all in super high detail with customization. The physics aren't
quite has hardcore as other racing games, but it's still very realistic. The game offers something for everyone, with easily set assists (for example, you can race with NO assists at all, or have the game do brakes and just about everything for you. Less assists = more money in events) and different types of events; autocross, bowling, car soccer, endurance races, demolition derbies, car tag, regular circuit racing, stock car racing, drifting, and drag racing.
Forza's editor lets you create unique vinyls for your car; some players drive around with recreations of the Mona Lisa on the sides of their car, or try to recreate real racing liveries.
The game starts you in a choice of European city cars (Toyota Aygo, Ford Ka, etc). As you rank up from doing races, it offers you a choice of free cars to own. Like at Driver Level 2, it'll offer you a choice of popular drifting and tuning cars (Toyota MR2, Nissan 240SX, VW Golf, etc).
Tripod and I have been playing it (with me whoopin his butt on HUMMER races on the Top Gear test track) quite a lot.
Also, Jeremy Clarkson provides commentary on some cars.





My own pictures:






Videos:
http://www.youtube.com/watch?v=uInIbD5KI1w - All the cars in the standard game (Missing the muscle car DLC)
http://www.youtube.com/watch?v=zAL3-FVoG48 - Muscle Car pack
http://www.youtube.com/watch?v=Km-8ZBDuf1Y - RUF Rt 12 S cockpit gameplay
http://www.youtube.com/watch?v=sTndj3dAs9s&feature=related - 1960 Corvette C1 gameplay (with an idiot driving)