I just downloaded an application called “LiveDictionary” that acts as a Safari extension to look-up words in a variety of dictionaries.

LiveDictionary offers a wide array of dictionaries for technical terms, general language, and bilingual usage.

And, as an example of ease-of-use:

To use LiveDictionary, point your mouse cursor to the word you want to look up.

It seems very powerful and could help non-english speakers a lot in browsing and understanding the web. If you want a solution that has instant lookups, I heavily recommend looking at LiveDictionary!