Pons has quickly become my online English-German dictionary of choice - it beats the others hands down. It has pronunciation samples, sample sentences, phonetic alphabet, ...
Pons picture lexicon (Bildwörterbuch)
...and a picture dictionary. Helps a lot with building up a vocabulary, an important part of a language learning.
[en] -> [de]
[de] -> [en]
Google translation, often quite wrong but still very useful
Pronunciation samples - very comprehensive
Contains translated sample sentences (they seem to be from EU documents).
Declination tables and some grammar
Deutsches Wörterbuch / German Dictionary
A German dictionary with untranslated sample sentences
Duden dictionary
Another good dictionary, although I don't use it as much
Has a lot of info in there, in a fiddly interface: German dictionary, untranslated sample sentences scraped from newspapers, German thesaurus, etymological dictionary, pronunciation samples, even statistics on how word usage changed throughout the century
Very popular English-German dictionary and forum, it does have some funny entries. Android app is useful.
Phonetics: The sounds of German
Old school Flash app with animated diagrams and pronunciation samples.
Easy online German - YouTube
A series of pronunciation sample videos put on youtube by a woman called Sonja. She's very thorough, and has easy to follow pronunciation tips
The Global Experience German guide
A series of pronunciation sample videos explained in German with subtitles. Not as good as Sonja's.
A complete German course like the commercial Rosetta Stone, but free.
About.com Learn German
German grammar and lessons
Deutsche Welle Learn Center
A complete German course with video lessons and tests
Wordreference forum
A forum, can be useful some times, although like in all forums there is some weird stuff in there