VocabForager is a browser extension built for people build their vocabulary for one, or several, languages.
One way that people can try to learn languages is by reading articles and tracking the words they don't know. VocabForager facilitates this technique by allowing users to highlight words/phrases on any web page, search them using an online dictionary/translator of their choice, and quiz themselves on the vocabulary. Users can later leave the page and come back, seeing the text they've highlighted before.
Please visit the extension's website for more information.