Rikaikun is a browser extension for Google Chrome that shows you all the possible meanings and readings of a word or kanji when you hover over it with your mouse. You can press shift or enter to switch between word, kanji, and name dictionaries while hovering. Rikaikun and Rikaichan (for Firefox) have long been must-haves for everyone from beginners to experienced learners of Japanese.
There are customizable options within Rikaikun as well, such as changing the color and position of the pop-ups, showing only readings without meanings, and choosing which of the thirteen available kanji dictionaries you'd like to see definitions from. The plentiful hotkeys, copy to clipboard (for flashcard use), and text-to-speech functionalities make Rikaikun an even more powerful tool for browsing the web in Japanese.