WordReference.com is my favorite online translation site. It has multilingual dictionaries to/from English and Spanish, French, Italian, German, Russian, Polish, Romanian, Czech, Greek, Turkish, Chinese, Japanese, Korean, and Arabic.
It has very active discussion forums for vocabulary, translation, and grammar for the languages listed above plus Catalan, Latin, Dutch, Hebrew, Greek, Hungarian, Finnish, Tagalog, and more. In addition, there is a forum for just English, for those who are learning the language; another for Spanish/French; another for French/Italian; and another for specialized Spanish vocabulary.
Another great feature of WebReference.com are the search results. They could include — sometimes from multiple sources — the translation for the singular word, the word as part of a compound term, the conjugated verb as part of an expression, thesaurus links, and, best of all, direct links to discussion items that have the word in the subject. These usually include questions and answers about words used in expressions or idiomatically.