Lingro is a site that offers a useful translation tool. Enter the URL of a site or page, then choose the language you want to translate words into and you can click on the words you don't know and bring up a pop-up box with a translation.
Here's a version of the homepage of this blog set to translate words into Spanish. It also allows you to save the words you have clicked on and to create lists of words that you would like to learn from them. There are also games and you can edit and add to the dictionaries. At the moment, they have dictionaries in English, Spanish, French, German and Polish, though I assume other languages will be added as it gets more popular. This is a site worth checking out if you speak one of these languages.