A brilliant (free!) site I came across a while ago and keep meaning to blog about is www.lingro.com . This site allows you to choose a dictionary (using wikitionary) and then a website you want to read. The website you want (www.yahoo.fr for example) then opens up in the lingro browser. Now each time you click on a word, you are given the translation. This is great because you still have the word in context which makes it a little easier to choose the correct translation.
Two other neat features are the fact that if you create a free account, you can log in and it keep a list of all of the items of vocabulary you have looked up and the second is that it will create (albeit simple) flashcard games with the aforementioned vocabulary in order to help you learn it. Great for independent learning especially in 6th form but also great as a support when working with authentic texts with younger year groups.