Animated German Grammar - PowerPoint presentations, many with sound.
Coloring Grammar Game – Spaß mit Wortarten
Cannonet - Deutsche Grammatik, Online Wörterbuch zur Rechtschreibung, Flexion und Wortbildung für die Sprache Deutsch.
Tiscali: German Phrase Finder – Grammar – Basic concepts.
Genetive Case – a simple way to explain how to use the genitive case.
Adjective Endings for "ein" Words– a simple, cut & dry method to teach adjective endings.
Internet Handbook of German Grammar - "This site is intended to provide concise and comprehensive reference grammar."
Grammar Memorization Songs – Simple songs to help students remember grammar rules etc.
Interactive Grammar Tutor - Summaries, detailed explanations, extensive examples, practice exercises and diagnostic exercises for all the basic aspects of German grammar.
Logos Universal Conjugator - verbs listed in alphabetical order and conjugated.
The inventor of the language seems to have taken pleasure in complicating it in every way he could think of. For instance, if one is casually referring to a house, Haus, or a horse, Pferd, or a dog, Hund, he spells these words as I have indicated; but if he is referring to them in the Dative case, he sticks on a foolish and unnecessary e and spells them Hause, Pferde, Hunde. So, as an added e often signifies the plural, as the s does with us, the new student is likely to go on for a month making twins out of a Dative dog before he discovers his mistake; and on the other hand, many a new student who could ill afford loss, has bought and paid for two dogs and only got one of them, because he ignorantly bought that dog in the Dative singular when he really supposed he was talking plural -- which left the law on the seller's side, of course, by the strict rules of grammar, and therefore a suit for recovery could not lie.
-- from "The Awful German Language" by Mark Twain
The Germans have an inhuman way of cutting up their verbs. Now a verb has a hard time enough of it in this world when it's all together. It's downright inhuman to split it up. But that's just what those Germans do. They take part of a verb and put it down here, like a stake, and they take the other part of it and put it away over yonder like another stake, and between these two limits they just shovel in German. - Mark Twain's Speeches, "Disappearance of Literature"
From Mark Twain: Quotes on the German language