My favourite German online dictionary is:

http://www.duden.de/

This is a good dictionay because it shows

1) How much a word is used

2) Meaning

3) Pronunciation

4) Declination (ex. Plural)

5) Verbs/Adjectives/Nouns used with this word

6) Examples

I am using this dictionary a lot at work, especially when I have doubts about the gener of one word, the plural or which verb should I use with word.