Nancy Thulens Webseite mit Grammatikubungen (fuer Grund- und Mittelstufe: sehr systematisch, und ausgezeichnet fuer das Einuben von allen wichtigen grammatischen Strukturen!)
http://www.4teachers.de/ You have to register on this site, but it is worth it. An incredible collection of short reading texts and exercises to go with them make it a helpful AP reading resource, but there’s grammar here as well. Go to Arbeitsmaterialien, then Deutsch. It would be nice to be able to contribute, but I haven’t figured out how to do that.
http://www.deutschlern.net/ Terrific advanced online exercises – great source for AP- type reading selections and listening texts spoken slowly and clearly.
http://www.interdeutsch.de/studien1.htm A variety of exercises for beginning, intermediate and advanced students. There are even two Vater und Sohn picture sequences to put in the right order.
http://www.stk.tu-darmstadt.de/jcal/ Advanced Grammar practice.
http://www.daf.uni-mainz.de/melk.htm These teaching units prepared by DAF students in Mainz focus on cultural themes as diverse as Umwelt, Erziehung and Burgen.