Even though I've lived in London since September, I seem to have missed Borough Market until just a few weeks ago. The market can be dated back to 1040 - so not quite the beginning of London, but close to it! Even more exciting, the market has actually been on the same street since the 13th century.
However, it's definitely one of my top markets I have been to- a foodie's paradise! It is essentially an enormous farmer's market (there are 130 regular stalls, plus different ones that rotate through). The market is filled with almost every gourmet food you could imagine. Plus, just like any good farmers market, lots of the food is produced locally, with fresh produce, fresh fish, butchers...plus a few surprising things I hadn't seen before.