One of the best websites I've seen, for the sheer amount of information it holds, the way it is organised and its overall design is Vincent Van Gogh: The Letters.
There is an exhibition in London at the Royal Academy of Arts until 18th April 2010. I hope to go to that the next time I'm in London, but for the moment the website gives me a fascinating glimpse into Van Gogh's life.
There is also a superb book of the letters but it costs £395, so thank goodness for the website.