Booking.com are the all-time kings of this approach. The search results page is littered with highlighted messages like.
“4 people are looking at this room now”
“2 people just booked at this hotel”
“Too bad you already missed out on this hot deal”
“X% of the neighbourhood/city/region you’re looking at is already booked!”
...
It’s quite likely real information, but the timeframes and language are vague enough to make you feel like you’ve got about 5 minutes to lock a room down. They do such a good job at instilling anxiety that you feel like you achieved something important when you finally book.