In addition to its standard dictionary, Jisho.org has a searchable collection of sentences. These can be useful for finding out the use of a word in context, or an English word that is represented instead as a suffix or particle in Japanese.
Hikyaku
The Japanese writer's dictionary, hikyaku will give you more than just the word you're asking for, it will give you several more specific terms, probably including the one you really want. For instance, a search for "Dog" returns Japanese for "puppy dog" "wild dog" "mutt" "police dog" the "Akita" breed of dog, a miserable, wretched life (a "dog's" life), "stray dog," "stuffed dog," and a dozen more, with Kanji for each. Not only that, but other words using these Kanji are also shown, leading to an explosion of vocabulary for a search as simple as "dog." Surprisingly, the direct translation for just "dog," was actually rather difficult to find among all the flowery vocabulary-boosters. For a similar but less overwhelming experience, try the English-Japanese Online Dictionary.
The English-Japanese Online Dictionary
The English-Japanese Online Dictionary returns multiple results, but they all appear on one page. For an easy themed vocabulary list, this dictionary may be helpful. It is interesting to note that this site has a host of other services for beginning Japanese scholars, ranging from prerecorded pronunciations of Japanese words to online quiz bowls to show off your knowledge, to a custom-made Japanese-teaching Manga. Take a look if you're a self-motivated studier!
Yamasa
Extensive kanji support is what you'll get at yamasa.org. Each Kanji has three large panels showing its print form, an animation of its stroke order, and the form it takes in handwriting. You'll also get the On reading, Kun reading, Joyo grade, and stroke count. Vocabulary buffs can search for words that start with a Kanji or simply contain the Kanji anywhere.
PSP Japanese-English Dictionary
In addition to some somewhat unsettling smiling, staring people, the Free Online Bi-directional Japanese-English dictionary will give you a list of useful words related to your search query. Good for less intense vocabulary-building. For the most rigorous vocabulary building experience try hikyaku.
WWWJDIC
JDIC has a number of specialist dictionaries from which to choose:
Computing/Telecomms
Life Sciences/Bio-Med
Legal Terms
Finance/Marketing
Buddhism
Miscellaneous
Special Text-glossing
Engineering/Science
Linguistics
River and water systems
Automobile Industry
It also supports translation to and from Japanese in Russian, French, German, and Swedish.
Jtango-Web
This dictionary is exceptionally easy to use. It accepts romaji and separates results and meanings into different columns. It also has a tag column that gives helpful information in relation to each word's use in modern Japanese dialect.
Tangorin
Tangorin has a set of specialist dictionaries as well as a dictionary of sentences for grammar.
The Forgiving Online Kanji Search
FOKS, or the Forgiving Online Kanji Search, claims to be smarter than the average Japanese dictionary. It returns a number of results, giving each one a rating according to its accuracy. People who often miss syllables (particularly long vowels) will appreciate this dictionary. It does not accept romaji, unfortunately, so you'll have to be able to type kana to use it.
Anime Lab
Who could forget Japans's number-one American fan base? Under the watchful eye of a randomly chosen anime character, you can search for words in Japanese or English, or you can browse by kana. Animelab features a large dictionary, from which a single request can get dozens of entries. Anime-lovers and Japanese vocabulary buffs are encouraged to try this one out.
Japanod
Japanod's defining feature is real-time search. As you type, you will see the list of words appear and change to match your search. It's fun to watch, but doesn't actually make looking up words much easier.
Spencer's Dictionary
Despite the slightly troubling name, spencer's dictionary has a wealth of vocabulary that dwarfs other dictionaries. It lists all the results and gives tips for where each stands in the Japanese common dialect. Excellent for vocab building.