In celebration of their 100th destination, Qatar Airways has a buy one get one free sale until tomorrow, April 14. The offer is good for roundtrip economy class travel between May 1 and June 14, 2011. Their routes to Sao Paulo and Buenos Aires are excluded, and travel originating in the United States must terminate beyond Doha. Full terms and conditions are available in the link above.
(Tip of the hat to Andre)