A couple car packs have come out in the last few weeks that I didn't see anyone mention (though I haven't made it to Forza night in a little while, maybe this was already covered).

1 - VIP car pack is now available if you bought the regular version.
QUOTE
'08 Aston Martin DBS, '09 Chevrolet Corvette ZR1, '07 Ferrari F430 Scuderia, '07 Lamborghini Gallardo Superleggera, and '07 Porsche 911 GT3 RS (997)

2 - The Jalopnik car pack was released today.
QUOTE
This pack contains ten cars: 2008 Mazda Furai, 2010 Jaguar #33 Jaguar RSR XKR GT, 2009 Ford #40 Robertson Racing Ford GT Mk7, 2010 Aston Martin V12 Vantage, 2010 Porsche 911 Sport Classic, 1992 Bugatti EB110 SS, 2011 BMW Motorsport Z4 sDrive35is, 2010 Audi TT RS Coupe, 2010 Aston Martin Rapide, 2010 Honda Mugen Civic Type-R 3D.