single leg squats are awesome for overall stability and building functional strength. i do them when i cycle away from heavy stable training. it really adds stability when you get away from traditional bilateral movements and go to single leg, balance types. once you come back to the standard, you feel rock solid.
ill use a single leg squat to assess muscle imbalances as well. excellent for that.
bodyweight squats can get you too, using a 4/2/2 tempo for 15-20 reps. imagine a set taking 2 min. ouchy. even moderate weight dumbbells will get the quads screaming.