Denshi Jisho — Online Japanese dictionary

Jisho.org extensions

KanJisho [English]
A Chrome extension for quickly looking up words in Jisho.org.

Dictionaries

Jim Breen's Japanese Page [English]
Home of the WWWJDIC dictionary server, its related dictionary files and a good list of links to pages related to Japan.
QuickKanji [English]
Beautiful and easy to use kanji dictionary with list functionality.
Jeffrey's Japanese⇔English Dictionary Server [English]
Very customizeable dictionary based on data from WWWJDIC.
Goo 辞書 [Japanese]
Online versions of the Daijirin Japanese-Japanese dictionary and the Exceed Japanese-English-Japanese dictionaries.
Yahoo 辞書 [Japanese]
Online versions of several Japanese-Japanese and Japanese-English-Japanese dictionaries.
英辞郎 [Japanese]
Japanese-English-Japanese dictionary. Available both online and on CD.
2典Plus [Japanese]
Japanese-Japanese dictionary of Japanese net slang.
Japanska.se [Swedish]
Japanese-Swedish-Japanese dictionary with user submissions and corrections.
Beyond Calligraphy [English]
A great, well researched and copiously illustrated guide to kanji and kana etymology.

Blogs on Japanese

Tofugu [English]
Wonky Japanese Language and Culture.
Nihongojouzu [English]
"Stuff for learning Japanese"
nihongo.3yen.com [English]
"Learn Japanese langauge and study Japanese grammar." By the same author as A Japanese guide to Japanese grammar.

Other useful resources

PopJisyo [English]
Feed it a website and then hover over a Japanese or English word to have a translation immediately pop up.
Rikai [English]
Same idea as PopJisyo with some extra features.
Rikaichan [English]
Extension for Firefox providing Rikai-functionality directly from the browser.
JGram [English]
User edited database of Japanese grammar.
Sci.lang.japan [English/Japanese]
The sci.lang.japan newsgroup. Interesting discussions about the Japanese language.
Sci.lang.japan FAQ [English]
A list of questions and answers about the Japanese language from the Usenet newsgroup sci.lang.japan.
日本語表現文型辞典 [Japanese]
Explanations and examples of almost 500 expressions.
Charles Kelly's Online Japanese Language Study Materials [English]
Lot's of online study material and a good collection of links to other Japanese resources.
A Japanese guide to Japanese grammar [English]
"This site explains Japanese grammar in a systematic step-by-step process /.../"
A Logical Japanese Grammar [English]
Explanations of Japanese grammar.
Four-Character Idiomatic Compounds [English]
Comprehensive list of over 3000 four character compunds (yojijukugo) with readings and English explanations.
Language Exchange Project [English]
"Language Exchange Project is a simple and intuitive website to find a language exchange partner, pen pal or friend."
Flashcard Tree [English]
Online English/Japanese flashcard (SRS, spaced repetition system) application with dictionary.