Whether or not you're using the Komm Mit! series, the publishers' website has a host of activities which will be useful for the first three years of German study. These are practice activities - you need to understand the grammar and vocabulary before you do them, but they'd be a valuable party of any home study course. Four stars.
Kathleen Pepin and Hank Schwab are two American language teachers who also provide links to materials for Komm mit! and also to activities based around various other courses and for general use. The pages aren't very attractive visually, but don't let it put you off. There's a lot of good stuff there. Three stars each.
Languages Online is a super site, mainly intended for 11-14s, but also very useful for older beginners. It starts from zero and has a step by step, graded approach which gives plenty of practice of one thing before moving on to the next. The site is produced by Andre Balaam, a British secondary school teacher, and many of the exercises are intended to supplement the Logo!series of coursebooks. The site is not particularly flashy, but it's well-organised and easy to use, and in terms of content definitely gets a five-star rating.
The Channel Four website offers Homework High, a service where kids of 11-19 can ask questions about German and get an answer from a teacher. Useful for home-schooling parents too, if you've got a doubt.