I bought several squares and rectangles that are sized to go together, which I will be using to "process" scraps. I bought rectangles that finish at 1-1/2x3, and 3x6, and squares that finish at 3", 4" and 6". I figure that gives me a nice variety for putting together scrappy 12" blocks.
I also have bought the circular rag-cut die that you use to make a sort of raggedy cathedral-window-ish block, and a long skinny"wedge" shaped die. I'm using the circular die now to cut raggy denim circles and putting fleece squares inside; I think that's going to turn out to be a warm and heavy quilt. The wedge I kind of bought on a whim, not 100% sure what I'll do with that one yet - probably more scrappy quilts. I just liked the shape!