http://www.rikai.com/perl/HomePage.pl?Language=Ja
http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1C
http://www.popjisyo.com/WebHint/Portal_e.aspx
http://www.sanseido.net/
(JPN-JPN dictionary - useful for those looking to study monolingually)
http://trans.glova.jp/
(sentence translation and usage tips - helpful for translation)