Google Chrome has a new add-on that translates random words in a webpage into your target language. unfortunately it doesn't work for japanese yet.
(article): http://www.theverge.com/2012/5/3/2995523...-extension
(download link): https://chrome.google.com/webstore/detai...hogkaggkhl

however, you can use the already existing add-ons called polyglot and langtest
https://chrome.google.com/webstore/search/polyglot

[s0apgun]

No Japanese though?

yeah, no japanese. i wonder why. i know that grammatically japanese is unique, but it could still be a useful vocabulary builder. polyglot supports japanese, but i don't imagine it is as well done as the google one. you need rikaikun with polyglot if you want to highlight the word and see its translation. polyglot does have an option to see the original word in parenthesis but i'd like to only see a translation if i need it.