Wörterbuch der deutschen Sprache des 20. Jahrhunderts (DWDS):
Dictionary of the German language of the 20th century
Deutsches Wörterbuch (DWD): German dictionary by Jacob &
Wilhelm Grimm (19th century): meanings, etymology & quotes (Wörterbuchnetz)
Free dictionary: Großwörterbuch Deutsch als Fremdsprache
Wortschatz: morphology, synonyms
OpenThesaurus: synonyms & meanings
Reverso Collins: German-English dictionary
Collins: German-English dictionary
Leo: German-English dictionary with pronunciation
Pons: German-English dictionary & French, Italian, Spanish, Polish & spelling
Bildwörterbuch: German-English visual dictionary
Linguatec: German-English dictionary
Wordreference: German-English dictionary
BeoLingus: German-English dictionary
Linguee: German-English translation of a phrase
Eurocosm: translation of a phrase in German, English, French ,
Italian, Spanish (with audio)
German-English online translation & other languages: texts & web
page
German-English-French-Italian dictionary
German multilingual dictionary
German vocabulary for beginners: by themes, with images
etymological dictionary of law
usual expressions German-English with sound (Loecsen)
Talk German & usual
expressions & vulgar with sound (BBC)
Basic German vocabulary (1991) 
Using German synonyms (2004)
1000 deutsche Redensarten: German expressions (1981)
Etymological dictionary of the German language by Friedrich Kluge
(1891)
Dictionary of quotations (German) by Lilian Dalbiac (1906)
Encyclopaedic English-German dictionary, unabridged
edition, by Eduard Muret & Daniel Sanders (1900)
English-German: A-K & L-Z & German-English: A-J & K-Z
Encyclopaedic English-German dictionary, abridged
edition for school and home, by Eduard Muret & Daniel Sanders (1910)
Dictionary of the English and German languages by Immanuel Schmidt, Gustav
Tanger & Felix Flügel (1901)
Nathan Bailey's dictionary English-German by Johann Anton Fahrenkrüger
(1801)
Old German & 19th century dictionaries
Meyers Konversationslexikon (1889)
world atlas: all countries
nature encyclopaedia: fauna, flora, minerals...
German keyboard to type the special characters of the German alphabet & punctuation marks
verb conjugation & German-English translation
noun declension, genders & numbers
German grammar with pronunciation (Deutschkurs)
German grammar (in
English or in German)
German grammar with dictionary (in German)
German grammar (in German)
German lessons & vocabulary (about)
German course [PDF] (Foreign
Service Institute)
Easy learning German grammar, Collins (2007)
Concise German grammar, Oxford, by Timothy Buck (1999)
Essential grammar of German
(1998)
German learner's guide, Berlitz (2003)
Essential German grammar (1961)
Deutsche Grammatik, ein
Handbuch für den Ausländerunterricht (1996)
Kurzgrammatik Deutsch: German
grammar (2008)
Grammar of the German language by George Curme (1922)
Beginners' German by Max Walter & Carl Krause (1916)
German grammar for beginners by Paul Valentine Bacon (1916)
German grammar for schools and colleges,
by W. H. Van der Smissen & W. H. Frasers (1910)
High school German grammar by W. H. Van der Smissen & W. H. Fraser
(1910)
German self-taught by Franz Thimm (1905)
Short comparative grammar of English and German as traced back to
their common origin and contrasted with the classical languages, by Victor Henry (1894)
German conversation-grammar by Emil Otto & William Cook (1887)
books about German language: Google
books & Internet archive
Eurotopics: bilingual news German-English & French
Arte: videos & news in German or in French
Deutsche Welle: online radio
videos with texts
Das Erste: online German television
Tageschau: news
ZDF: news
Schweizer Fernsehen: Swiss television
German press & international
books about German literature: Projekt Gutenberg & Google
books & Internet archive
Erlkönig by Johann Wolfgang Goethe: the most beautiful German poem
Grimm's fairy tales (Märchen der Brüder Grimm)
Grimm's fairy tales: selection of tales, with sound & translations
song lyrics in German & other languages, Christmas carols
Bible: German-English & other languages, Luther Bible (1545)
Alle Menschen sind frei und gleich an Würde und Rechten geboren.
Sie sind mit Vernunft und Gewissen begabt und sollen einander im Geist der Brüderlichkeit begegnen.
Germany: maps, symbols, heritage & documents
Low German language