I discovered Open Food Facts very recently. I was supersurprised because the mobile app is very neat, and I didn’t expect there would be so many products (edit: in Spain). I’ve sent two contributions so far.

Also, you can download their database. If I had some time, I’d try to run some queries on it. (I’m on a low sodium diet and sometimes you find the most unexpected products with little salt, but it’s time consuming.)

edit: also, I forgot, the app is on F-Droid, another nice touch.