1. Linguee - http://www.linguee.com/
Linguee is the right word for good translations. The dictionary Linguee is used by millions of people. They also offer smartphone apps for iPhone and Android for offline use.
2. Leo.org - http://www.leo.org/
Leo.org is a popular online German to English dictionary that is heavily used in Germany for the translation of English words. By registering for my LEO (also entirely free) you can save vocabulary words to practice later.
3. PONS - http://en.pons.com/translate
This popular dictionary features a clear and simple design. When looking up words, translations as well as phrases containing the search phrase are shown.
4. DICT.CC - http://www.dict.cc/dict/options.php
The goal of dict.cc is to make it possible to share your vocabulary knowledge with the world. This is the main difference from other translation services - every user is encouraged to contribute to the English-German dictionary by adding and/or verifying translation suggestions. The resulting vocabulary database can be downloaded and used for free.
5. Dict.tu-Chemnitz.de - http://dict.tu-chemnitz.de/
Powerful dictionary that has the added benefit of having plenty of phrases and examples to go along with each word. You may find this one to be easier to handle than Leo, as it is more forgiving if you forget to add an umlaut or even spell the word incorrectly.
6. Bab.la - http://en.bab.la/dictionary/german-english/
Search results are very easy to navigate and always come with synonyms, examples and context. On top of this very solid and beautifully designed dictionary, bab.la offers many other helpful features such as a conjugation browser, quizzes, grammar overview and games.
A simple but useful dictionary that displays word definitions and translations along with common phrases and expressions.
8. OpenThesaurus - https://www.openthesaurus.de/
A useful reference work that provides you with synonyms and associated words to your search term along with other useful information.
9. Google Dictionary (for chrome) - https://chrome.google.com/webstore/detail/google-translate/aapbdbdomjkkjkaonfhkkikfgjllcleb
Google Dictionary has a lot of different languages with features like voice pronunciation, definitions, example sentences, related phrases, related phrases and more.
10. IM Translator (for firefox) - https://addons.mozilla.org/En-us/firefox/addon/imtranslator/?src=search
IM Translator leaves a pretty solid impression with more than 21 Million downloads and a four-star review.