The remarkable site called POPjisyo.com describes itself as “a web based pop-up dictionary for Japanese, Chinese, Korean and other languages.” I found it via mj klein of Metrolingua (an excellent language-oriented site), who says:
What you do is paste in Japanese words in a text box to look up both the meaning and the correct reading of the word, plus the correct reading of the individual kanji. For instance, if you see a word that contains two or more kanji (such as 国連）but don’t know the correct reading, just type in the kanji, press “Word Lookup” and it will provide the meaning of the word. Plus, when you move the cursor over each kanji, it will give you all the readings and meanings of each one (like a dictionary).
But that’s not all–you can also create your own study list. All you do is double click on the word, and it will add it to your list, and every time you go back to the site, your list is sitting there, waiting for you. And, if that’s not impressive enough, you can email your study list to yourself! I have at least 50 words on my list, which is also sitting in my email inbox.
You can also create a bookmarklet that will allow you to go to, say, a Japanese site and find the meaning of any word by holding the cursor over it. Amazing.