OS Neutral Racing Games with real cars?

As real car brands as it gets right now. There are also F1 2012 and Dirt 2, though not on Steam.

Racedriver: Grid (on the Mac app store, not steam) also has real world cars if I remember correctly. It's retail predecessor Toca Race Driver 3 is also a really good game and has loads to do. It might not look so hot by todays standards but it plays well and is huge.

I've only played the xbox 360 version od Grid but it was good fun and pretty exciting. I like how the opponent AI make mistakes and you get some spectacular crashes.

F1 2013 on Steam.

if you're running steam on windows 7, asseto corsa is a good choice if you want a sim. There is always iracing too. This is assuming you like sims of course