If you purchased a Kindle book from Hachette, HarperCollins, Simon & Schuster, Penguin, or Macmillan between 4/1/10 and 5/21/12, you should have some free money sitting in your Amazon account right now.
If you're a consumer, odds are you can get money back as part of many class action lawsuits (like this week's big Red Bull suit). Top Class Actions lists current settlements so you can see what you might be eligible for.
A class-action lawsuit against Red Bull is being settled, and if you've purchased any Red Bull products in the last 12 years, you can get a piece of the pie. No proof of purchase required.