Welcome to the Quilting Board!
I have used the Wash Away sheets by C & T Publishing. I really liked them. They are a bit on the spendy side but they save a big chunk of time by eliminating the need to remove them. I used them for a wall hanging and once I was done piecing the top, I just rinsed it in the sink under running water. 90% of the stiffness was washed out. I didn't mind a little stiffness for a hanging though so didn't worry about washing the finished quilt. I imagine after a couple washings, the stiffness would have been completely gone.
I'm not totally sure but I think I just bought some Soft-Fuse from my LQS. I think that is what the lady told me it was called. It's really thin, has a paper back and is supposed to be soft right away after fusing. It came on a big bolt but doesn't have any writing on the paper side. I haven't used it yet though. I must have figured I was going to like it though because I bought three yards! :)