nihongoresources.com - more than just a dictionary site
In addition to Japanese dictionary functionality, this website offers free online lessons in Japanese at University level, as well as a free grammar book and several articles of interest to people learning Japanese.
The search system should be fairly self-evident; JP search terms are treated as full terms, letting you use wildcards if you need start/substring/end matching. Both * and ? are allowed; * in your search text means "zero or more letters", and ? means "exactly one letter". These wildcards are context sensitive, so if you're looking for なに? then the ? is considered a hiragana syllable, and if you're looking for 一生*命 then the * is considered one or more kanji. English searches are "natural text" searches, and can do with some improvement, but I'll need your opinion on the best approach: tweet at me with what you think would work best!