A tweet from @2nihon the other day alerted me to tangorin, another online Japanese dictionary that I was not aware of. There are quite a few online dictionaries out here, for a variety of languages. Some of them are not so good but there are some excellent dictionaries for Japanese that not only provide meanings, but also example sentences, kanji and kanji stroke order.
If you're looking for good Japanese online dictionaries, these ones work for me...
Denshi Jisho - Good simple to use dictionary. Will accept words in roomaji or kana script. Usually provides useful example sentences to help get the meaning you are looking for. Whatsmore, it automatically optimizes for the iPhone or iPod touch, making it easy to use on these devices.
Tangorin - Although I haven't used this one much as yet, it seems quite comprehensive and simple to use. It allows you to search for words, kanji, examples and even names or classical Japanese. It also gives you the option to sign up, create a profile and build vocabulary lists. One of the best things I have found for this site is the ability to search for different verb forms. If you enter a verb form it will put it into example sentences for you, thereby allowing you to better understand the structure and how it is used.
Jim Breen's WWWJDIC - A fairly comprehensive dictionary that provides meanings, examples and even links to google, google image or wikipedia results for the Japanese word. It is not really visually appealing in its layout but can provide some very useful information on words or phrases.
I haven't used EnglishJapaneseOnlineDictionary for a while but did use it with classes a while back. It could be better than it was before.
For something different, try one of these:
Although it is a little hard to navigate around, Wapedia has a variety of lists. Lists of abbreviated words like ファミコン (famikon), English words of Japanese origin and gairaigo or words that have originated in languages other than Japanese, like Dutch and Portugese. For example, did you know the Japanese word for 'swing' (ブランコ) comes from the Portugese word? Or, the word ランドセル (randoseru), meaning backpack / knapsack, comes from the Dutch word.
Another interesting site is Rikai. It has a cool little tool that allows you to hover over a kanji or word in the page, revealing the reading and meaning for the kanji or word. If you type the address of a Japanese site into the 'box' on the front page it will redirect you to that site and again allows you to place the cursor over a word to reaveal its meaning. Although not 100% effective, it is pretty good.