AllWords.com
Type in a word in one of six languages and find out its translation in any of the others. Also look for words that end in or contain a particular combination of letters.

Diccionario de Inglés-Español
Type in a word and find the Spanish equivalent or its definition in English.

Diccionario Logos
Dictionary puts words from numerous languages in context. More suited for browsing than word-finding, this dictionary is for lovers of words and languages.

Diccionarios.com
Thorough definitions for translating between Spanish and English. This would be the best Spanish-English dictionary on the Web if it were more reliably available.

ES English-Spanish Dictionary
Type in an English word and find out the Spanish equivalent.

The Internet Dictionary Project
Here's an interesting concept: Have Internet users create a dictionary by adding words to it. Although it's a work in progress, the Spanish vocabulary appears to be quite large.

Online English to Spanish to English Dictionary
Get translations not only of the word you type in, but of related words as well.

SpanishDict.com
Type in a word and get the translation in Spanish or English. A nice feature is that sound clips provide the pronunciation for many words.

WordReference.com Best of the Net
One of the best dictionaries for finding idioms and phrases. Type in "town," for example, to find out how to say "to go out on the town" in Spanish.