Duden: Online dictionary of the German language
German-English Dictionaries
Grammar Guides
Canoo: German grammar guide, provides tips on word structure, sentence grammar, spelling and more
Grimm Grammar: Practice your grammar while interacting with 36 Grimm Brothers' fairytale characters.
Nancy Thuleen's Grammar Guides: Personal website providing a variety of German grammar tools
Deutschkurs: An online manual for the German language, built for English speakers