I'm slowly turning into an insomniac. It's driving me crazy, but on the other hand check out what I found five minutes ago!
http://quizlet.com/13731924/scatter
I tried googling stuff like "list of the most difficult words" and I accidentally came across this great site. I don't want to waste too much time googling cute kittens or other silly stuff (Internet is a time-consuming creature!), that is why I started to google some linguistic-related features, Enjoy!