This is a great Chinese newspaper site.
Definitions of words pop-up when you move the mouse over the text.
Why can't we do this in English? Do we have a good enough public domain internet dictonary? Select the "About" link to read the description of the software which includes an online vocabulary learning program for students. The Chinese dictionary is open source and can be actively improved and added to by students.
Online Glossing is a program that can be used online to add mouse-over vocabulary definitions to a web page.