http://www.tiscali.co.uk/reference/dictionaries/difficultwords/

Update: Site no longer functions – no way back machine link as it wasn’t good anyway

Pretty sure this site quietly died. So in it’s place I’d recommend the Oxford Dictionary of Difficult words.

Available (of course) from Amazon:

The original site I reviewed on StumbleUpon wasn’t that good as it didn’t have disestablishmentarianism or antidisestablishmentarianism.