I find that static lunges and rear lunges are easier on the knees than forward lunges (and the risk of putting pressure *forward* on the front knee is greatly reduced). Also, doing lunges with the front foot on a platform (4"- 8") helps put the emphasis into the glutes and hamstrings and out of the knee.
Also, when doing any kind of lunge, make sure that your knees are not just remaining behind the front toe, but also not buckling inward or flaring outward.
You could also hold onto a support (a post or beam around the house, a doorway, and lunge backward from there. This can both keep your knee from moving forward (and hitting the support) and help you put the weigth back and down instead of forward).
Whatever modifications you choose, remember, if it hurts don't do it. Your body is your best guide as to what works for you.