Agreeing with Carbonfiberboy, but you can also add in single legged squats with weights and lunges also work well. I've been training a lot of my clients with kettlebell training, and there are some great kettlebell exercises that will help work on strength development and power training.
It could also be leg length differences. I went to one workshop with Joel Friel and they broke out two identical scales. We stood on both simultaneously, and you could see if the scales came out even or if there was one side that weighed in heavier. It's a decent tool to spot check if you have the leg length issue or if you tend to favor one side over another.
koffee