Wesley's Chapel is the mother church of Methodism, built by John Wesley and still one of the most important Methodist sites in the world. Wesley preached here, lived next door, and is buried behind the chapel.
The site includes Wesley's House and the Museum of Methodism, which give the visit more depth than a quick look at the chapel alone. It is a good Clerkenwell stop if you are interested in London dissenters, social reform, or religious history beyond the Anglican mainstream.