Great question. I've been finding my best deals on Priceline, though my guess is there are probably better deals to be had if you know what you are doing.

I haven't used Hotwire, though it looks like it is worth checking out. Priceline has some "anonymous" deals where they don't reveal the name of the hotel until after you book, but I've found that if you hunt around you can sometimes figure out which hotel it is based on number of stars, original price, amenities, etc. Sometimes it is even actually stated if you read the fine print before you book.

Then there's also the "just show up" method. It was probably 20 years ago now that I was making a spur of the moment trip to Yosemite, and called in the morning to book a hotel just outside the park. The guy who answered the phone earned my eternal gratitude (and hopefully some good karma) by telling me not to bother making a reservation on the phone, but to just show up and walk in. There was hardly anyone staying there (it was winter) and their "walk-in price" was lower. When I got there, it turned out to be half what I would have paid if I had booked on the phone. A couple nights later I moved to a different hotel but still nearby, and had a similar experience with the walk in price being much lower. I've tried that a few other times elsewhere in the last couple years, but without success. The prices I've been quoted walking in have seemed to be if anything higher than what I would pay booking online.