LiveDictionary and dict.cn
As far as I know, there is not an online dictionary that can give the meanings of the words as the cursor moves over them. But the "LiveDictionary" can do it.
LiveDictionary is a small software which works with Safari (the web browser). And you can download many dictionaries for this software. One of those dictionaries is "CEDIT english -> chinese dictionary".
There are two website that provide some free dictionary services: "iciba.com" and "dict.cn".
I recommend the dict.cn, they provide a search-word link that you can drag it into your browser's bookmark bar. After that, You can hightlight a word in a webpage, and use the keyboard shortcut to search this word. (If you dragged the link on the first position of bookmark bar, then your keyboard shortcut would be apple-1.)