Chrome: Surrounding yourself as much as possible with a new language you want to master is one of the keys to learning a language quickly and fully. Language Immersion for Chrome gets you one step closer by switching select words on web pages into your target language.
When turned on, the extension automatically translates parts of the web page into one of 64 languages supported by Google Translate (Japanese being the big exception). Novice, intermediate and fluent level settings control how much of the page is translated (on fluent, for example, entire sentences are translated, while on novice, only a a few phrases per paragraph or page). Rolling over a translated word or phrase lets you hear it pronounced, and clicking the translation turns it back into English.
One of the best things about the extension is that you're learning new words while reading topics you're already interested in. Use it on web pages whose content you're quite familiar with (for example, content you wrote or an article you've read more than once in English) and you'll be able to recognise the meaning of unfamiliar words very quickly. Even on unfamiliar pages, though, puzzling out the translations in the context of all the other English words is a great way to learn.
The translations aren't 100 per cent accurate, so you'll need to take them with a grain of salt (for example, the translations might use the wrong gender or the more literal/formal versions of words and phrases). Still, the extension is a pretty awesome tool for building your vocabulary, learning pronunciation, and surrounding yourself with more reading material.