When I've run out of laundry detergent and I have some shampoo around I will use the shampoo in place of my regular laundry detergent when needing to wash clothes without having laundry detergent.
The shampoo works just as good as my laundry detergent to get the clothes clean.
You can also use baking soda in place of laundry detergent or even some dish detergent such as Dawn or other dish soap when you've run out of your regular laundry detergent.