My new favorite site is Autoslash.
I made a rental car reservation in Florida at Christmas and Thanksgiving, to visit family. Hertz gave me a price over $700 for one and $500 for the other. I reserved. But then I entered my reservation details into Autoslash, and it has been automatically emailing me as prices fall, allowing me to re-book, at about $200 in both cases. So savings of $800. The same happened last year with a New York rental (though those miracle prices took many more weeks to appear).
I have no idea how they make money. I suspect they don’t.
In case you are wondering, this is an unsolicited endorsement. I’m just ecstatic that I saved $800 for five minutes of form filling, and figure the frequent travelers that read the blog will be eager to do the same.