This is one of those topics where I realise how limited I am for 'free' food whilst living in suburbia... I used to be able to get my hands on blackberries, about five different types of mushrooms and all the rabbits I could be bothered shooting, but no more.
However I still have access to a heck of a lot wild greens, including dandelions, nettle, wild lettuce, plantain, several different species of thistle (in particular, I like sow thistle), wild brassicas and fat hen. Of course I need to make an educated guess about where I harvest them from, but they are nutritous, pretty much free, and very fresh. I am currently in the UK and I am amazed how much wild forage there is only a short distance out of London... on my 2mi walk today I encountered an abundance of dandelion, apples, blackberries, nettles, plantain, thistle and acorns (although I have played with making acorn flour, it's just too much work). I am amazed that people let free things like this go to waste... nettle gnocci is lovely!