You should post your question on the videofitness.com General Discussion board (http://forum.videofitness.com/forumdisplay.php?s=512095b0ff8d3c799c010b07a280ee0d&f=3). A lot of users on that forum use Rebounders along with cardio DVDs (not necessarily designed for Rebounders) with excellent results.

Do you have room for an exercise bike? Very easy on the knees.

You can get a mini-tramp at Target. There's some that suggest it's easier on the knees, e.g.:
http://www.examiner.com/article/best-cardiovascular-workout-for-knees-and-joints-is-rebounding-on-a-trampoline

In any case, if it doesn't work, you haven't invested too much $.

After many, many years as a runner, I've been forced to give up running because of osteoarthritis of the knee. I'm usually able to get a pool workout in once or twice a week, but I'm looking for a home exercise routine as well. Most exercise DVD's are too heavy on the squats and lunges for my knees. I'm wondering if a rebounder (mini trampoline) might be a good way to get a little cardio in without killing my knees. Does anyone have any experience with these, or have other recommendations for cardio that's easy on the knees? I can do about 20 minutes on the elliptical without too much pain, but I really need something I can do at home.