1. Useful iPhone apps
Midori
Japanese (yes, white kanji on red icon)
Wisdom
G4 Genius dictionaries
This set will pretty much do exactly what most EJD's do.
(They also have others:
http://www.mrbass.org/iphone/japanese/)
In addition, through a web browser:
www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1C
www.microsofttranslator.com
www.jisho.org
http://kotobank.jp/ http://www.alc.co.jp/ these will provide machine translations and access to eijiro.
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What will an EJD provide you?
Only the JUMP feature and multi-word example search.
The JUMP feature in most good EJD's will look up that word or kanji across all dictionaries.
The multi-word search will let you look up things like "catch & cold" and it'll bring up sentences that have "Don't catch a cold." and the equivalent Japanese.
However, with WWWJDic Example Search, you can get the same thing.
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EIJIRO covers the latest slang and so forth not covered in most EJD's. You can get it for a PC - most Japanese bookstores sell a book + CD-ROM of EIJIRO if you don't want to download it.
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EPWING on any PC/Laptop + EPWING dictionaries all over the net will give you everything an EJD has as well.
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MIDORI on the iPhone is the ONLY dictionary I've found that has it's own Japanese Kanji lookup pen input pad. Yes, iPHone has Chinese character written input pad built-in, but IT WON'T look up any of the Japanese characters that aren't in CHINESE!!! (iPhone is so sad - 4th gen and still no decent handwritten Kanji input)
JAPANESE is nice because it lets you build flash cards easily.
Both of the above are based on EJDICT (wwwjdict site above), so they're not the same with definitions as either WISDOM or GENIUS dictionaries (not necessarily better - just different sources).
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Realistically, once you have an iPhone, no real need to spend $$$$ for an EJD at all. Just spend $ on a few Japanese English dictionary apps and you'll be learning in no time.
ALSO, many Learning Japanese apps as well that'll get you to past basic grammar, etc.
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FINALLY, VoiceTRA!!!!! From the Japanese government.
Speak or type English or Japanese, get back Japanese or English (or the many other languages supported).
Good for basic sentences and words, but very nice for travel and such for a quick translation of a simple sentence.