The Flewent language learning plugin takes a webpage in english and translates some of the words into a language of your choice.
This should help you build your vocabulary for a foreign language you want to learn while reading news, checking wikipedia, and doing daily web browsing. It also provides a pop-up to view the word on Google Translate (for better / more results), Wikipedia, or Dictionary.com
Recent versions made substantial changes to the way the plugin works. It now includes your own dictionary that fills up automatically and lets you translate some words without using the translation service. Your dictionary will fill up from the translation service, but you can edit the dictionary to suit your needs. Full release details here: