House Mill was built in 1776 and is the biggest tidal mill in the world and, while a lot of the equipment including some of the grinding stones remain on the site, it is no longer operational due to the most of the equipment being sold for scrap when milling ended in the 1940s due to bomb damage to the surrounding buildings. The exterior is very fine though, as is the Clock Mill next door. The island also happens to be the early home of London gin, as a lost of the grain that made its way onto the island was used to ferment alcohol in the distillery just on the other side of the river Lee. Interestingly, with changes being made to the nearby waterways in preparation for the 2012 London Olympics, this may provide enough of an increase in the flow of water past the mill to make it viable again
Just as an aside, you may also have heard of Three Mills Island because this was the original home of the Big Brother House, which sat on a back lot of the vast Three Mills Studios. It's long gone now though - when they originally built it they were under strict instructions to return the land to its previous condition when they moved out, so all thats on the site now is a patch of scrub land with no indication of its previous notoriety.
