I can refer the USA Starwood Personal American Express

Earn 25,000 Starpoints after making $3000 in purchases within 3 months (plus another 3000 points earned on your spending). Now transferable to Marriott points.

Personally I think this is better than the 2 free nights offer. Points don't expire if you keep your account active but the other free nights offer is only good for 12 months.