iOS/Android/Web: If you wait until the last minute to make a reservation at a popular destination, most hotels will be booked. If you have your heart set on a certain location, Hotel Room Alerts will text or email you if a room becomes available.
After you pick the hotel and dates you want, they'll tell you the availability. If the room you want is available, they'll switch you to a booking site. If not, just give them your email or phone number, and when that room is available, they'll let you know.