Both Kalsoom(Chup) and the good folks at All Things Pakistan have compiled a list of organizations you can donate to. If you've already donated and can give more, please do so. If you are in Pakistan and in close proximity to any of the areas that are affected, please volunteer. The victims of these floods need all the help they can get.
Just to give you an idea of how bad it is, here is the before picture (from NASA via Tazeen @ twitter) of Sindh and Punjab