Buy 2 get 2 nights at Marriott.

Marriott is offering a free night to new members who stay two nights.

http://www.marriott.com/rewards/promotion.mi?promotion=FN12

If you have an account, you can sign up a household member or make yourself a new account.

This promotion is stackable with the Megabonus, which also gives you buy two get one.

http://www.marriott.com/rewards/promotion.mi?promotion=MS14

Together, you buy two nights, you get two nights free, quite nice, as the free nights can be in any category four hotel, even if the ones you bought were lower.