Brush-up on essential German grammar concepts with the Hammer Grammar series of online lectures, made to be useful as well as entertaining (and at times quite silly). The videos are made by Catherine Liggett, in collaboration with OSU Ecampus.
The series of over 30 videos follows the structure of the Berliner Platz NEU (1, 2, and 3) textbooks, but is entirely free and usable for anyone.
Click here for the entire YouTube playlist, and make sure to bookmark the page for future reference.
Sample video on modal verbs: