Also, watch for the third or fourth night free offers if you care about elite night credits. Sometimes the free nights don't count, yet you might be able to find a regular rate giving approximately the same total cost that would give credit for all nights.

I've seen some fourth night free offers that allowed eight, twelfth, etc. night free too, all on a single reservation.

They're not consistent and some agents don't know the rules (or are told the rules incorrectly).