Actually, I can't imagine a barbell exercise that couldn't be done with dumbbells. And in many cases, at least with the upper body, the exercise would be more effective because you have to call some stabilizing muscles into play, and you work both sides equally. There is a comfort advantage to using a barbell for squats and such, because you can rest the bar on the back of your shoulders/traps, but if you can find a comfortable place to hold dumbbells (at the shoulders, in a "hammer grip" --I can never put them ON my shoulders: OUCH!-- at your sides, etc.)then that takes away the advantage.
As for lower weight/higher rep workouts: PH, ME, Leaner Legs.