振り仮名Furigana is a Japanese reading aid, consisting of small phonetic characters printed above a word with Chinese characters to indicate the characters' pronunciation (as seen to the right). It's used in materials for native-Japanese kids who are still learning the characters and in materials for native-Japanese adults for very difficult characters, but it also has obvious uses for language learners.
Furigana Inserter is an extension for Chrome and for Firefox that will let you add furigana to any open webpage. With Furigana Inserter, you can turn this:
So as you can see, it's a pretty useful extension for a Japanese learner. Once convenient use for this is adding pronunciation info to dictionaries that don't have them out of the box (ALC, I'm looking at you).
The following instructions lay out the simple steps to get it up and running in Chrome and the not-so-simple steps to get it up and running in Firefox on a Mac.