> It's a pain trying to filter them out of the rest of the 100 varieties.
Arguably, shops specifically design themselves to do things like this. When I was growing up, it used to drive my dad mad that the local supermarket would move almost literally everything around seemingly at random "to prevent shoplifting". He was a pilot, and usually had little time -- he liked to go into supermarket, grab x, pay, and leave. Suddenly, that stoped working, as finding x became increasingly difficult. It really annoyed him. For a long time the floor manager said that they moved their stock around to prevent shoplifting – until one day the store manager, when asked the same question, said it was "to promote footfall". Suddenly the real reason became clear.
>Another thing I would like to do is compare products by price/quanity. Maybe I'm trying to get the most protein/$, or oz/$, etc. You could also hook it into some database to only show you ethically caught fish, organic projects, fair trade products, local products, etc.
In the UK at least, all of this information is required to be printed on the label: product per unit money on the shop label and misery per unit product on the product's label.
Are you asking for a filter / search engine for that on top? Either way, I don't think your average supermarket would let that stay un-advertised for too long. Products at eyeball level pay a premium to be there.