What are good rehab exercises for a torn knee ligament?

Careful. Depends on the ligament. It is very important to get your motion back first after a ligament injury. Range of motion exercises (wall slides, etc.) and biking without resistance can accomplish this. Then if you are treating the ligament nonop, strengthening exercises are begun. For example- PCL injury, the quad is very important. Ask your doctor for specific goals for your specific ligament injury.