I like https://paribus.co/
It automatically requests refunds for things you buy online that drop in price. It's nice because it's very passive and it's basically found money when it works.
Worth noting that Amazon recently discontinued its practice of supporting these refunds, which was a major use case of this app. If you're someone like me who primarily is buying from Amazon, this won't help you on that.
Seems a scumbaggy app to me (even if it's legal). If you buy something for $X, it's because you can afford $X at that point in time and you agree that that's its value.
Many credit cards offer "price protection" or something where if the price drops with X days after you buy something they'll refund you the difference. It doesn't seem scumbaggy to take advantage of a service a company offers you.