Leo English/German dictionary
This is a German/English and English/German dictionary with about 260,000 entries. It's hosted by Technische Universität München.

English/German dictionary, proverbs, aphorisms, quotations, explanations, synonyms
Here you'll find a German/English and English/German dictionary with about 130,000 entries. It also provides proverbs, aphorisms, quotations, explanations and synonyms. It's hosted by Technische Universität Chemnitz.

Word Reference.com
It's a German/English and English/German dictionary. It also provides French and Italian to English and vice versa. You can download translation plug-ins for Netscape Navigator 4+ and Internet Explorer 4 or 5 on Windows 95/98/NT for free.

PONSline
This is a German/English and English/German dictionary. It also provides French, Italian, Polish and Spanish translations.

phrasen.com
An excellent collection of English and German sayings, phrases and expressions.

Merriam Webster Online
It's an English/English dictionary. They offer the Collegiate Dictionary and also the Collegiate Thesaurus online. Additionally you will find audio pronunciations.

Wordsmith
The Wordsmyth Educational Dictionary-Thesaurus is an online American English dictionary with an integrated thesaurus.

Roget's Thesaurus
They offer several services: dictionary and thesaurus.

yourDictionary.com
All in all you will find links to over 800 dictionaries in about 150 languages. The English part also offers a thesaurus.

xrefer
Calls itself "the web's reference engine". xrefer contains encyclopedias, dictionaries, thesauri & books of quotations. All entries are cross-referenced.

Cambridge Dictionaries Online
You'll find lots of sample sentences and grammatical information here.