In this question, I first heard of opaque hotel booking sites. It sounds very interesting, but I don't know such a site. So my question is: Where should I start to look for opaque hotel booking sites?
An "Opaque" booking is one where you don't know the exact details of what you're booking until after you've made the booking. The most common form of opaque bookings are for hotels, but they also exist for flights.
There two general formats of booking sites for opaque hotels - those that work on a "bidding" concept, and those that have a fixed price.
Priceline was the first (and still best known) of the "bidding" types of sites. For their bidding site, you specify an exact date range, a general vicinity where you want to stay, a star range (eg, 3 star or higher) and a price. If your "bid" matches something they have available then you will "win" that hotel - however you will not be given an exact details of the hotel (including the name or exact location) until after you've placed your bid - and by that stage you are not able to back out of the booking.
Fixed price sites include Hotwire's Hidden Hotels, Wotif's "Wot hotel?", Travelocity's "Top Secret Hotels", Expedia's Unpublished Rate hotels, and many more. With these you are shown details of specific hotels, but without the exact hotel name, location or amenities. eg, you might be told it's a 3.5 star hotels on the Las Vegas Strip with a pool. Sometimes you can reverse engineer these descriptions back to a specific hotel (eg, for many countries simply Googleing the description from Wotif's hotels will let you work out the exact hotel!), but more freqently you are not given enough details to do so.
For both bidding and fixed price, the moment you make the booking you are given the details of the hotel - including name, address, etc. From that point you can't generally make any changes to the booking or cancel it - you'll be charged the moment you make the booking and it's non-refundable.
Almost without exception, booking a hotel via an opaque method will result in you paying less, but with the uncertainly that you don't fully know what you're going to get. You will also normally not earn any form of loyalty points/status when booking via an opaque channel (ie, frequent flyer points, hotel points, etc). There's a good explanation on why deals such as this exist in the answers to this question on opaque flight bookings.
There are a number of web forums, such as Bidding For Travel, where people share tips for booking opaque hotels, including details of prices paid and hints to help maps descriptions to specific hotels.
I googled "secret hotels" and easily found a bunch. Here are two:
The going rate seems to be a 50% discount, or so. This is steep, but will still not likely beat regular B&Bs, or AirBNB.