Summary
Denshi Jisho is a free online Japanese dictionary that leverages the powerful WWWJDIC Project to provide users with a way to find words, kanji, kanji by radicals and even sentences. It is very lightweight and loads quickly!
The site has a version that is optimized for iPhone and iPod Touch.
A forum is also available for discussions about Japanese words, kanji and more.
From Website
Denshi Jisho is an easy-to-use and powerful online Japanese dictionary. It lets you find words, kanji and example sentences by searching in many ways.
The dictionaries are also interlinked so that you can check what the kanji in a word mean individually or what context a word can be used in. You can also look up kanji by the parts they contain.
The data is publicly available and comes from the excellent WWWJDIC project.
Denshi Jisho Bookmarklet: Drag the link to the favourites/bookmark bar in your browser. Select a Japanese word on a page, then click the bookmarklet to make a quick lookup on that word.
For keitai usersDenshi Jisho is also available for Japanese mobile phones. Just go to jisho.org with your keitai and it should automatically take you to the mobile version. You can also use k.jisho.org to access it from any device.
For iPhone users: By going to jisho.org with your iPhone or iPod Touch you will automatically get Denshi Jisho optimized for those devices. You can also access it from any device by going to iphone.jisho.org.
For western mobile usersYou can also access a light version of Jisho.org for western mobile phones. Just go to m.jisho.org. This requires that your phone has support for Japanese fonts, so it might not work for everyone.