Jim Breen's Japanese Page http://www.csse.monash.edu.au/%7Ejwb/japanese.html
This page has a wealth of information and links to dictionaries, information about translation software, information about reading Japanese WWW pages using a browser without Japanese fonts.

Jim Breen's WWWJDIC Server: Japanese-English Online Dictionary (Monash University Site) http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1C

Babylon http://www.babylon.com/define/110/Online-English-Japanese-Dictionary.html
Provides English-Japanese, Japanese-English dictionaries online and for downloadings as well as some more specialized dictionaries: computer terminology, terms associated with continuous improvement, marketing and transport. When relevant, the search also goes to the Japanese wikipedia.

Denshi jisho  http://www.jisho.org/
Words, kanji and sentences can be searched in either English or Japanese. Includes a bookmarklet which you can drag to the favourites/bookmark bar in your browser and then select a Japanese word on a page, then click the bookmarklet to make a quick lookup on that word.

Goo jisho http://dictionary.goo.ne.jp/index.html

Jeffrey's Japanese/English Dictionary Server http://linear.mv.com/cgi-bin/j-e/sjis/dict

Plant Name Database http://www.plantnames.unimelb.edu.au/Sorting/Frontpage.html
Several indexes of plant names categorized by scientific classification, but yielding common names in English, Chinese, Japanese et al. Pictures will be included where possible. Botanical expertise not required.

Rikai http://www.rikai.com/
Allows you to paste a URL or block of text into the translator, and it provides the Japanese reading and English translation for words. Does Japanese to English, English to Japanese, English to Chinese, and English to Spanish.

Sanseido web dictionary http://www.sanseido.net/Index.aspx
A searchable dictionary site -- includes kokugo, ei-wa and wa-ei dictionaries.

Dictionary translation sites: Excite http://www.excite.co.jp/world/url/ and  Altavista (Babel Fish) http://babelfish.altavista.com/
Type in a URL and press go.