It is a very small, simple to use Dictionary which I have been using for more than 4 years.
You will have to download a small binary and the program will go and sit in your system tray.
Whenever while surfing/reading documents you come across a new word, all you have to do is select the word and click on the icon. A simple box will pop out and will give the meaning of the word.
BTW for the environmentally conscious, Morgan Stanley has printed a nice green book which is worth a peep.