This isn't a website, but I really learned a lot from the book "An Everlasting Meal: Cooking with Economy and Grace". It made me a much more frugal cook and grocery shopper. It also freed me from recipes, for the most part.
While the book has a lot of good recipes in it, its real genius is that it gives you lots and lots of examples for breaking free of recipes and just using what you have, and - importantly for readers of this site - using all of what you have.
For example, buy a head of broccoli and roast the florets. You're not done yet; Now core the stem, boil it and mash it with walnuts, garlic, Parmesan cheese, and olive oil. Now you have a pesto.