Sign up here for a free $50 voucher for Lufthansa for flights booked until May 1, 2011.
You must fly by August 1, 2011 on flights operated by Lufthansa, Lufthansa Italia and Lufthansa Regional Partner (incl. Lufthansa City Line and Eurowings) originating from the US.
UPDATE: Reader Bart pointed out that it is not only for the USA. When you follow the link, it asks for your country and will give you a voucher amount in your local currency. For example, Belgium was 40 Euro. Eligible countries are: USA, Germany, UK, France, Switzerland, Spain, Italy, China, Netherlands, UAE, India, Austria, Belgium, Singapore, Canada, Turkey, Russia, Hong Kong, Denmark, Portugal. More details are here here.