Re: alternative to barbell squats???
I highly recomend walking lunges as mentioned by Moxie - and changing the length of the stride can vary the emphasis between the quads and glutes.
I also do a one legged squat/lunge with one foot behind me on a bench. With one foot out in front of you, next to a wall or piece of equipment that you can use with one hand for balance, place the toes of the other foot on a bench behind you. Keeping your front shin at 90 degrees to the ground, squat down untill your back knee is near the ground and come up again. Do a set of 12 and change legs, repeat 3 times. This is a killer on the quads and as you progress, start holding a dumbell in the other hand.