Free Online Dictionaries
One of the most comprehensive and useful bilingual online dictionaries, this project initiated by Munich University offers a English/German/English and German/French/German dictionary as well as a forum for both languages where users can ask questions and have an active exchange of ideas.
Another useful, but not quite as comprehensive, bilingual dictionary for German/English/German. This Web site also has its own forum for users to exchange ideas and experience.
Wörterbuch der TU Chemnitz
Yet another good English/German/English dictionary.
A commercial, fairly comprehensive dictionary for English/German/English and French/German/French
Merriam-Webster's Collegiate Dictionary
This site contains only the abbreviated version of Webster's, one of the best monolingual dictionaries there is, but it still comes in very useful.
Sorted by categories, this incredibly comprehensive dictionary "portal" offers specialty dictionaries on almost any topic. Whether it be Accounting, Botany or Chocolate - there is hardly anything that cannot be found at YourDictionary.com. But the portal also has links to language dictionaries from Abenaki to Zulu, multilingual dictionaries on a large number of general or specialist topics such as Agriculture, Education, Internet, or Urban Planning, a page for links to thesauri, rhyming lexicons etc, ('nyms and such), "Today's Word", a section on word games (gameroom), information on endangered languages, and much more.
A highly interesting "mixed bag" of monolingual dictionary, thesaurus, a section on English grammar and style, language and grammar FAQs, the "word of the day", a list of links to other dictionarys (some of them are no longer available, though) and - if you feel like having a good laugh - a translator tool for quick (and often wrong!) automatic translation of various languages into one another.
Cambridge Dictionaries Online
This Web site contains several monolingual dictionaries, e.g. Cambridge Advanced Learner's Dictionary, Cambridge Learner's Dictionary, Cambridge Dictionary of American English, Cambridge Dictionary of Idioms, Cambridge Dictionary of Phrasal Verbs.
Ein- und mehrsprachiges allgemeinsprachliches Online-Wörterbuch mit Thesaurus.
Ein allgemeinsprachliches Online-Wörterbuch (einsprachig Englisch) mit Begriffsdefinition, Synonymanzeige und Konkordanz (vollständige Sätze, in denen der gesuchte Begriff erscheint).
(Die unten angezeigte Mehrsprachen-Funktion ist irreführend, da diese kostenpflichtig ist. Nur das einsprachig englische Wörterbuch ist wirklich kostenlos.)