Sites on INTERNET:
Goo dictionary – The J-E and E-J is probably suitable for beginner to intermediate Japanese, but as your understanding of Japanese improves, you should be able to make use of the kokugojiten which is incredibly useful.
Space ALC – Contains J-E/E-J dictionary which is incredibly useful for help with the language as commonly used: rather than focus on individual words, it’s packed with a range of example sentences and translations thereof.
Kenkyusha Online Dictionary – The Kenkyusha J-E dictionary, known to some as “the Green Goddess”, is considered by many to be absolutely essential for anyone interested in serious translating. Of course, it doesn’t come cheap. However, you can avoid breaking the bank by using the online version, which is available on a subscription basis. (I use the traditional offline version.
Nihongo zokugo jisho – A dictionary of modern Japanese slang.
Books made of “paper”:
Nihongo bunkei jiten, ISBN 4874241549. This is a dictionary of Japanese grammar patterns (as the title suggests). The grammar patterns are listed in gojuuon order, and each have a few example sentences and a short explanation in Japanese. Quite handy as a to-hand reference, I think, but remember that it IS a reference, not a book to study from. This book is aimed at both people studying Japanese and the Japanese themselves, so the example sentences have furigana for all the kanji, and the explanations are easy to understand.
Making Sense of Japanese: What the Textbooks Don’t Tell You, ISBN 4770028024. Despite the title, this is a great book which intermediate to advanced students would do well to read. Not only do the contents do a great job of illuminating areas of Japanese people might find confusing, but it’s written in an informal and friendly style that makes it easy and fun to read. The chapter on wa and ga is particularly enlightening. This book is also by Jay Rubin, who has written some great translations of some of Haruki Murakami’s work.
Kenkyusha Japanese-English Dictionary, ISBN 4767420164. See the description for the Online Dictionary above.
I wrote a couple of articles on Japanese grammar for everything2. I’ve reproduced them on this page; see the links in the column to the right.