Jim Breen of Monash University in Australia began work on a voluntary project to produce a freely-available Japanese/English Dictionary in machine-readable form in 1991. The result of this work is the Edict dictionary file, a plain text document in EUC-JP coding, which has involved the voluntary help of hundreds of people. Numbering over 70,000 entries, Edict is the major free-ware Japanese-English lexicon, and a great help when perusing Japanese texts. Although not Public Domain, it is free for personal use.
For more information, visit:
http://www.csse.monash.edu.au/~jwb/edict.html
A web-based search engine: http://www.csse.monash.edu.au/~jwb/wwwjdic.html
There is a similar project for the German language, called WaDoku.