There are many good online dictionaries for German learners. The top few are those I highly recommend for both their usefulness and their interfaces. Dictionaries with more specialized uses follow in alphabetical order. All of the dictionaries in this list are of good quality, and some might appeal to individual learners more than others. Try them out, and see which ones work for you!
An Encyclopedic Dictionary of Language and Languages Ref P29 .C68 1992
Routledge Dictionary of Language and Linguistics Ref P29 .B875 1996
The World's Major Languages Ref P371 .W6 1987
Der deutsche Wortschatz nach Sachgruppen Ref PF3591 .D6 2004
This compilation of German vocabulary lists words in subject categories.
Der Duden in 10 Bänden Ref PF3625 .D84 1988
The Duden is the standard reference work on the German language. Especially helpful for language learners are the following volumes: Stilwörterbuch (thesaurus) and the Aussprachewörterbuch (pronunciation dictionary).
Wörterbuch Deutsch als Fremdsprache Ref PF3625 .K46 2000
This dictionary is compiled for learners of German and contains good definitions and information on pronunciation and usage.