I'm a beginner of German and I wonder whether there is any dictionary as important as the Webster in English, which I found is extremely useful when I was preparing my GRE test.
There is only one monolingual German dictionary of substance and importance, the Duden.
Looking at my 3-Volume Webster I'm afraid, there is currently no counterpart (observing the decline of printed reference works like Britannica and Brockhaus this is not going to improve). In my opinion the closest match is the big Wahrig (ISBN 978-3577102414). In contrast to Webster it has nearly no diagrams of the objects described, just textual explanations, but it is not difficult to find the former in Wikipedia. At all occasions tried I was disappointed by German Wiktionary, if you want online information concerning word frequency and in which combinations a word is most likely used I recommend Deutscher Wortschatz by Leipzig University.
I think this one is pretty useful: http://de.pons.com/%C3%BCbersetzung/englisch-deutsch Hope it helps.