On-line dictionaries are very handy and the best thing is they are free. Since there are various sites available, try finding the one to meet your needs.
Jim Breen's WWWJDIC Japanese-English Dictinary Server (Monash University Site). The feaures inlude "Tranlate Word in Japanese Text" "Search for Words in the Dictionary" and so on.
A very useful tool. You can either type in Japanese, or type in a URL of a Japanese page. As you mouse over the text, the meanings of the words pop up in front of the text.
Rikaichan (I love the name!) is also available as a pop up Japanese-English/German/French/Russian dictionary extension for Firefox.
The customizable dictionary base on data from WWWJDIC (above).
Similar tool to Rikai (above). POPjisyo is a web based pop-up dictionary for Japanese, Chinese, Korean and other languages.
Since the translated words are shown in romaji, no knowledge of Japanese writing is necessary.
English-Japanese dictionary for English speaking people. All kanji include furigana.
A simplified version of Jefferys Japanese-English Dictionary Server (above).
Since Romaji is not accepted, you need to type the words in Japanese.
You can look up the words by glossaries of geography, travel, history, traditions, manga, video games, politics and media.
Japanese site. Online versions of the Daijirin Japanese-Japanese dictionary and the Exceed Japanese- English-Japanese dictionaries.