1. Tae Kim’s Japanese guide to Japanese grammar (link)
A good guide about Japanese grammar; this is where I refer to first if I need help in this area. Has a forum where you can ask questions as well.

2. JGram (link)
Another resource for learning grammar. Entries are sorted by JLPT level, and you can sign up for the grammar-a-day feature to receive entries in your email.

3. WWWJDIC (link)
Online dictionary, with translation and kanji lookup features as well. Example sentences are given for some words, and a list of verb conjugations can be found when searching for verbs. Also provided are links to the search term in ALC and Japanese Wikipedia. I use this site a lot.

4. Rikaichan Firefox plugin (link)
A very handy Japanese-English plugin. When activated, hovering over a Japanese word will bring up its meaning in English. Other than WWWJDIC, this is the other dictionary I use the most.

5. Goo Jishou (link)
Online Japanese-Japanese, Japanese-English, English-Japanese and idiom dictionary. Has a Firefox plugin as well, that allows you to search the dictionaries directly from the search bar in Firefox.

6. Japanese slang dictionary (link)
Search directly or browse by characters. Explanations are in Japanese.