Excellent quality: Pexels, Life of Pix, Stocksnap.io, Unsplash, Gratisography (by Ryan McGuire).
Good quality: Cupcake.
Good, but… a bit complicated to browse: Negative Space.
Great shots food, but the licensing is not exactly clear: Foodies Feed.
They are all free to use via the Creative Commons Zero license (aka CC0). Even for commercial projects. That means attribution is not required, so you can place the attribution where it makes sense and doesn’t mess with your website’s layout and design. Great stuff — truly free! 🙂
Sources: Slideshare blog and others.