Fels Naptha Soap bar - under $1 a bar!
Second this :)
I make my own laundry soap using Fels Naptha and rarely ever have to pretreat anything. Just finely grate 1/2 bar, add 1 cup borax & 1 cup washing soda. Use 1 Tbls per load.
Super cheap and works great even for my sensitive skinned girls.