Marriott corporate rate price tracker
This free website tracks availability for Marriott stays using corporate codes as opposed to the rack rate. If you are using the MMP Associate code, I recommend searching one day at a time if you are 90 days out or more (MMA code searches can be set for 120 days). The reason is some hotels often release inventory for associates at 90 days and once they are booked, the rate jumps. So, if you book a room one day at a time on exactly the 90 day mark, you can sometimes get a far superior rate. If you book multiple nights this way, you can reach out to the hotel and have the nights merged together so that you don't get moved to a different room. Here are some screenshots of the rather simple interface:
