Marriott Rewards is again offering a MegaBonus that rewards guests with a free night for every two nights spent in a Marriott property between September 15, 2011, and January 15, 2012. There are, of course, a few stipulations to keep in mind.
First, you need to be a Marriott Rewards member. Members earn points for every paid stay at any of Marriott's properties.
Second, you need to register on the Marriott Rewards website by October 15, 2011.
Third, you need to stay two nights between the dates listed above and pay with a Visa. Marriott Rewards has a Visa card that earns points for every purchase, plus a pretty big bonus for the first purchase.
If all of the requirements are met, you get a free night in a category 1-4 hotel to spend by May 31, 2012. There is a FAQ's section on the registration page.