http://life.gaiam.com/article/best-cardio-exercises-bad-knees
Hello
I have a knee issue as well (right side), the only impact free exercise I found is the elliptical machine, you could also do swimming but you can't do these at home, unless you buy an elliptical machine at home? that is what I just did and I am planning on start using it...You could use aerobic DVDs at home but you want to avoid jumping or moves that put pressure on your knee.
Good luck,