My MSc thesis was on exactly this - adaptive web sites. It's a fascinating research topic.
Got a distinction for it though! You can read a little about it here: http://www.puremango.co.uk/2009/06/adaptive-web-sites/
you can highlight words and it will show a "question box" that you can click to see more information about the word, google search, etc. it's displayed inline so you don't need to navigate away.
it's a nice idea, but it's terribly annoying.