I use Jim Breen's Online Japanese Dictionary Service a lot to help me look up words I don't know yet. A lot of the time, I believe it is capable of a little more help than the root of the word, but sometimes not. I use it either by typing in Japanese romaji in "Word Search/Home" or copy pasting kanji into "Translate Words". It doesn't do a very good job of typing in English words and finding a good Japanese equivalent though, so I wouldn't rely on it for that... Also, for "Translate Words" you still need to know grammer because it will only tell you the meanings of individual words in a sentence, not the meaning of the sentence itself! Anyway, I hope it helps!
WWWJDIC: Word Search
If you don't remember the site address, you can also just google "Jim Breen" and it's always the first link
